Make T440p Coreboot build depend on blob files

Now, when you run `make BOARD=any-t440p-variant`, the build system
automatically fetches mrc.bin and me.bin.

boards/t440p-maximized/t440p-maximized.config

@@ -35,9 +35,20 @@ export CONFIG_BOOT_DEV="/dev/sda1"
 export CONFIG_BOARD_NAME="ThinkPad T440p-maximized"
 export CONFIG_FLASHROM_OPTIONS="-p internal"
 
-all: $(board_build)/$(CB_OUTPUT_FILE)
-	@sha256sum $@ | tee -a "$(HASHES)"
+# Make the Coreboot build depend on the following 3rd party blobs:
+$(build)/coreboot-$(CONFIG_COREBOOT_VERSION)/$(BOARD)/.build: \
+    $(pwd)/blobs/haswell/mrc.bin $(pwd)/blobs/t440p/me.bin
+
+$(pwd)/blobs/haswell/mrc.bin:
+	COREBOOT_DIR="$(build)/$(coreboot_base_dir)" \
+		$(pwd)/blobs/haswell/obtain-mrc $(pwd)/blobs/haswell
+
+$(pwd)/blobs/t440p/me.bin:
+	COREBOOT_DIR="$(build)/$(coreboot_base_dir)" \
+		$(pwd)/blobs/t440p/download-clean-me $(pwd)/blobs/t440p
 
+# Haswell boards have an 8 MiB and 4 MiB SPI flash chip. So, we split the
+# Coreboot ROM into two files to flash one on each chip.
 all: $(board_build)/heads-$(BOARD)-$(HEADS_GIT_VERSION)-bottom.rom
 $(board_build)/heads-$(BOARD)-$(HEADS_GIT_VERSION)-bottom.rom: $(board_build)/$(CB_OUTPUT_FILE)
 	$(call do,DD 8MB,$@,dd of=$@ if=$< bs=65536 count=128 skip=0 status=none)