arm: ls1021atwr: Add distro boot support

This patch includes common config_distro_defaults.h and
config_distro_bootcmd.h for u-boot enviroments to support distro boot
which automatically scan boot.scr from storage devices(e.g.
SD/USB/SATA/SCSI disk) and execute autoboot script on LS1021ATWR board.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@nxp.com>
Signed-off-by: Alison Wang <alison.wang@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>