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
3 files changed