Add rule to get board value

In old design, it's hard to differentiate the product name via
TARGET(BOARD), so add a new rule to get board value.

Now the BOARD is assumed to be same as TARGET_BOOTLOADER_BOARD_NAME

Change-Id: Ifd95cb1c7d1834dfd23f84ee0f0b0bbb6d448f0f
diff --git a/makefile b/makefile
index 1e818a8..542bfaf 100644
--- a/makefile
+++ b/makefile
@@ -146,7 +146,7 @@
 
 # add some automatic configuration defines
 DEFINES += \
-	BOARD=$(PROJECT) \
+	BOARD=$(BOARD_NAME) \
 	PROJECT_$(PROJECT)=1 \
 	TARGET_$(TARGET)=1 \
 	PLATFORM_$(PLATFORM)=1 \