[Core] Allow usage of AVR minimal printf library

Enable linking of AVR's libc minimal printf implementation which
can shave ~400 bytes. Usually we use the xprintf implementation
but keyboards that use s(n)printf automatically pull in the AVR
libc implementation, which is ~900 bytes heavy.

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

docs/squeezing_avr.md

@@ -30,6 +30,19 @@ MAGIC_ENABLE = no
 These features are enabled by default, but may not be needed. Double check to make sure, though. 
 Largest in size is "magic" -- the QMK magic keycodes -- which control things like NKRO toggling, GUI and ALT/CTRL swapping, etc. Disabling it will disable those functions.
 
+If you use `sprintf` or `snprintf` functions you can save around ~400 Bytes by enabling this option.
+```make
+AVR_USE_MINIMAL_PRINTF = yes
+```
+
+This will include smaller implementations from AVRs libc into your Firmware. They are [not fully featured](https://www.nongnu.org/avr-libc/user-manual/group__avr__stdio.html#gaa3b98c0d17b35642c0f3e4649092b9f1), for instance zero padding and field width specifiers are not supported. So if you use `sprintf` or `snprintf` like this:
+```c
+sprintf(wpm_str, "%03d", get_current_wpm());
+snprintf(keylog_str, sizeof(keylog_str), "%dx%d, k%2d : %c");
+```
+
+you will still need the standard implementation.
+
 ## `config.h` Settings
 
 If you've done all of that, and you don't want to disable features like RGB, Audio, OLEDs, etc, there are some additional options that you can add to your config.h that can help.

platforms/avr/platform.mk

@@ -30,6 +30,15 @@ CXXFLAGS += -fno-exceptions -std=c++11
 
 LDFLAGS +=-Wl,--gc-sections
 
+# Use AVR's libc minimal printf implementation which has less features
+# and thus can shave ~400 bytes. Usually we use the xprintf
+# implementation but keyboards that use s(n)printf automatically
+# pull in the AVR libc implementation, which is ~900 bytes heavy.
+AVR_USE_MINIMAL_PRINTF ?= no
+ifeq ($(strip $(AVR_USE_MINIMAL_PRINTF)), yes)
+	LDFLAGS += -Wl,--whole-archive -lprintf_min -Wl,--no-whole-archive
+endif
+
 OPT_DEFS += -DF_CPU=$(F_CPU)UL
 
 MCUFLAGS = -mmcu=$(MCU)