ODROID-C5: env: improve boot commands to boot from SD or eMMC

This commit removes the hardcoded MMC device number and replaces it to
read 'boot.scr' in available SD or eMMC in a priority manner.

Signed-off-by: Dongjin Kim <tobetter@gmail.com>
Change-Id: I9857bbb708f1c2ad40c100131e447c6e42bb4d9f
2 files changed