sm8x50: Enable booting from mmc-sdcard

Set TARGET_SDCARD_BOOT=true at build time to build and boot
AOSP from mmc sdcard. This device configuration need atleast
16GB sdcard to boot from.

Booting from external sdcards will help prevent the internal
emmc/ufs wear off in the long run and extend the lab-life of
SM8550-HDKs.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Change-Id: I59530d39915c051565e9d02087164c1cbd1fbae2
diff --git a/sm8x50/sm8x50.mk b/sm8x50/sm8x50.mk
index 55f8476..d48f645 100644
--- a/sm8x50/sm8x50.mk
+++ b/sm8x50/sm8x50.mk
@@ -17,6 +17,9 @@
 
 $(call inherit-product, device/linaro/dragonboard/sm8x50/device.mk)
 
+#Boot from mmc-sdcard
+TARGET_SDCARD_BOOT ?= false
+
 # Product overrides
 PRODUCT_NAME := sm8x50
 PRODUCT_DEVICE := sm8x50