lk: Add generic trigger support for eMMC boot

For targets that support eMMC bootup, defining
TARGET_USERIMAGES_USE_EXT2 will cause an eMMC apps
bootloader to be generated along with the NAND flash
apps bootloader.
diff --git a/makefile b/makefile
index 5773c58..7ea95ad 100644
--- a/makefile
+++ b/makefile
@@ -37,6 +37,9 @@
 INCLUDES := -I$(BUILDDIR) -Iinclude
 CFLAGS := -O2 -g -fno-builtin -finline -W -Wall -Wno-multichar -Wno-unused-parameter -Wno-unused-function -include $(CONFIGHEADER)
 #CFLAGS += -Werror
+ifeq ($(EMMC_BOOT),1)
+  CFLAGS += -D_EMMC_BOOT=1
+endif
 CPPFLAGS := -fno-exceptions -fno-rtti -fno-threadsafe-statics
 #CPPFLAGS += -Weffc++
 ASMFLAGS := -DASSEMBLY