i.MX6UL: isiot: Switch the mmc env based on devno

Add board_mmc_get_env_dev

Switch the mmc env based on the mmc devno, instead of separately
defining a config item in include/configs using board_mmc_get_env_dev
- devno 0: sd/esd
- devno 1: mmc/emmc

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>