geam6ul: Add modeboot env via board_late_init

Add runtime, modeboot env which is setting mmcboot, or
nandboot based on the bootdevice so-that conditional
macros b/w MMC and NAND for CONFIG_BOOTCOMMAND should
be avoided in config files.

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