armv8: fsl : create bootcmd and mcinitcmd as per boot source

NXP platforms expect custom bootcmd and mcinitcmd to be
updated as per boot source with default environment.
Check env variable fsl_bootcmd_mcinitcmd_set to prepare
bootcmd and mcinitcmd

Fixes: cbf77d20 (armv8: fsl-layerscape: Fix automatic
setting of bootmcd with TF-A)

Signed-off-by: Wasim Khan <wasim.khan@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>