board: tbs2910: Autoselect environment device when booting from SD

Implement board specific functions to select the environment device and
partition when booting from SD/MMC.

SD2:  mmc 0 0
SD3:  mmc 1 0
eMMC: mmc 2 1

Signed-off-by: Soeren Moch <smoch@web.de>
2 files changed