ARM: imx: Add support for reading out the primary/secondary bmode

Add new 'getprisec' subcommand to 'bmode' command, which sets the return
value of the 'bmode' command to either 0 if the system booted from primary
copy or to 1 if the system booted from secondary copy. This can be used
e.g. in 'test' command to determine which copy of the system is running.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>