usb/gadget: fastboot: add eMMC support for flash command

- add support for 'fastboot flash' command for eMMC devices

Signed-off-by: Steve Rae <srae@broadcom.com>
diff --git a/common/Makefile b/common/Makefile
index aca0f7f..443e54c 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -264,4 +264,9 @@
 obj-y += memsize.o
 obj-y += stdio.o
 
+# This option is not just y/n - it can have a numeric value
+ifdef CONFIG_FASTBOOT_FLASH_MMC_DEV
+obj-y += fb_mmc.o
+endif
+
 CFLAGS_env_embedded.o := -Wa,--no-warn -DENV_CRC=$(shell tools/envcrc 2>/dev/null)