Add target 'check-md5' to build_keyboard.mk (qmk#11338)

* Add target 'build-for-compare' to `build_keyboard.mk`

The `build-for-compare` target provides an easy way to check the md5 checksum of the generated binary.

You can easily see if there is any change in the generated binaries between the two versions, as in the example below.

```
$ git checkout 0.11.0
M	build_keyboard.mk
M	tmk_core/rules.mk
Note: checking out '0.11.0'.
HEAD is now at c66df16 2020 November 28 Breaking Changes Update (qmk#11053)

$ make helix:all:build-for-compare | grep ^MD5
MD5 (.build/helix_rev2_default.hex) = 5c3606562c944bb4d18832e601b45d4a
MD5 (.build/helix_rev2_edvorakjp.hex) = 9e43d13d389d518ba7e99cd7337e28d6
MD5 (.build/helix_rev2_five_rows.hex) = 8bcb61c2fd5d237c2997f2fa007d4934
MD5 (.build/helix_rev2_five_rows_jis.hex) = b97cd818d52f73ca2d4e78c86d90a791
MD5 (.build/helix_rev2_froggy.hex) = c492172364188f4e2918b10bf0f3a0a6
MD5 (.build/helix_rev2_froggy_106.hex) = b0861fd735a8f81881a8c02730641a2b
MD5 (.build/helix_rev2_led_test.hex) = 5c97d982a5da5cfb3dacb28a8934b81d
MD5 (.build/helix_rev2_xulkal.hex) = 01f603dc46bcf9094d7e106831d8f5b1
MD5 (.build/helix_rev2_yshrsmz.hex) = 5a008bca2d0c5790a151c02834c529ba

$ git checkout 0.11.1
M	build_keyboard.mk
M	tmk_core/rules.mk
Previous HEAD position was c66df16 2020 November 28 Breaking Changes Update (qmk#11053)
HEAD is now at cc08e30 nix-shell: add milc dependency (qmk#11086)

$ make helix:all:build-for-compare | grep ^MD5
MD5 (.build/helix_rev2_default.hex) = 5c3606562c944bb4d18832e601b45d4a
MD5 (.build/helix_rev2_edvorakjp.hex) = 9e43d13d389d518ba7e99cd7337e28d6
MD5 (.build/helix_rev2_five_rows.hex) = 8bcb61c2fd5d237c2997f2fa007d4934
MD5 (.build/helix_rev2_five_rows_jis.hex) = b97cd818d52f73ca2d4e78c86d90a791
MD5 (.build/helix_rev2_froggy.hex) = c492172364188f4e2918b10bf0f3a0a6
MD5 (.build/helix_rev2_froggy_106.hex) = b0861fd735a8f81881a8c02730641a2b
MD5 (.build/helix_rev2_led_test.hex) = 5c97d982a5da5cfb3dacb28a8934b81d
MD5 (.build/helix_rev2_xulkal.hex) = d848383adfd7463b138c6da179cf1436
MD5 (.build/helix_rev2_yshrsmz.hex) = 5a008bca2d0c5790a151c02834c529ba
```

* make builds reproducable by default

* update build_keyboard.mk: remove 'build-for-compare' target

* GNU make (3.81) on macOS 10.14(Mojave) does not have the 'undefine' directive.

* Adopted fauxpark's suggestion.

* Update tmk_core/rules.mk

Co-authored-by: Ryan <fauxpark@gmail.com>

* update tmk_core/rules.mk

* fix tmk_core/rules.mk

Co-authored-by: Zach White <skullydazed@gmail.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

build_keyboard.mk

@@ -368,6 +368,7 @@ endif
 
 build: elf cpfirmware
 check-size: build
+check-md5: build
 objs-size: build
 
 include show_options.mk

tmk_core/common/command.c

@@ -144,10 +144,8 @@ static void print_version(void) {
     print("VID: " STR(VENDOR_ID) "(" STR(MANUFACTURER) ") "
                                                        "PID: " STR(PRODUCT_ID) "(" STR(PRODUCT) ") "
                                                                                                 "VER: " STR(DEVICE_VER) "\n");
-#ifdef SKIP_VERSION
     print("BUILD:  (" __DATE__ ")\n");
-#else
-    print("BUILD: " STR(QMK_VERSION) " (" __TIME__ " " __DATE__ ")\n");
+#ifndef SKIP_VERSION
 #    ifdef PROTOCOL_CHIBIOS
     print("CHIBIOS: " STR(CHIBIOS_VERSION) ", CONTRIB: " STR(CHIBIOS_CONTRIB_VERSION) "\n");
 #    endif

tmk_core/rules.mk

@@ -79,7 +79,12 @@ CSTANDARD = -std=gnu99
 #  -Wall...:     warning level
 #  -Wa,...:      tell GCC to pass this to the assembler.
 #    -adhlns...: create assembler listing
-ifndef SKIP_DEBUG_INFO
+DEBUG_ENABLE ?= yes
+ifeq ($(strip $(SKIP_DEBUG_INFO)),yes)
+  DEBUG_ENABLE=no
+endif
+
+ifeq ($(strip $(DEBUG_ENABLE)),yes)
   CFLAGS += -g$(DEBUG)
 endif
 CFLAGS += $(CDEFS)
@@ -110,7 +115,7 @@ CFLAGS += $(CSTANDARD)
 #  -Wall...:     warning level
 #  -Wa,...:      tell GCC to pass this to the assembler.
 #    -adhlns...: create assembler listing
-ifndef SKIP_DEBUG_INFO
+ifeq ($(strip $(DEBUG_ENABLE)),yes)
   CXXFLAGS += -g$(DEBUG)
 endif
 CXXFLAGS += $(CXXDEFS)
@@ -140,7 +145,7 @@ CXXFLAGS += -Wa,-adhlns=$(@:%.o=%.lst)
 #  -listing-cont-lines: Sets the maximum number of continuation lines of hex
 #       dump that will be displayed for a given single line of source input.
 ASFLAGS += $(ADEFS)
-ifndef SKIP_DEBUG_INFO
+ifeq ($(strip $(DEBUG_ENABLE)),yes)
   ASFLAGS += -Wa,-adhlns=$(@:%.o=%.lst),-gstabs,--listing-cont-lines=100
 else
   ASFLAGS += -Wa,-adhlns=$(@:%.o=%.lst),--listing-cont-lines=100
@@ -202,7 +207,10 @@ REMOVEDIR = rmdir
 COPY = cp
 WINSHELL = cmd
 SECHO = $(SILENT) || echo
-
+MD5SUM ?= md5sum
+ifneq ($(filter Darwin FreeBSD,$(shell uname -s)),)
+  MD5SUM = md5
+endif
 
 # Compiler flags to generate dependency files.
 #GENDEPFLAGS = -MMD -MP -MF .dep/$(@F).d
@@ -431,6 +439,9 @@ check-size:
 	$(SILENT) || echo "(Firmware size check does not yet support $(MCU) microprocessors; skipping.)"
 endif
 
+check-md5:
+	$(MD5SUM) $(BUILD_DIR)/$(TARGET).$(FIRMWARE_FORMAT)
+
 # Create build directory
 $(shell mkdir -p $(BUILD_DIR) 2>/dev/null)