i.MX6Q: icorem6_rqs: Add modeboot env via board_late_init

Add runtime, modeboot env which is setting mmcboot based
on the bootdevice so-that conditional macros for MMC via
CONFIG_BOOTCOMMAND should be avoided in config files.

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>