arm64: ls2088ardb: Add distro boot support

arm64: ls2088ardb: Add distro boot support

Include common config_distro_defaults.h and config_distro_bootcmd.h
for u-boot enviroments to support automatical distro boot which
scan boot.scr from external storage devices(e.g. SD/USB/SATA/SCSI disk)
and execute autoboot script. Tested on ls2088ardb with automatically
boot Ubuntu from SD card or USB disk, if it fails to detect external
storage disk, fall back to nor/qspi boot.

Signed-off-by: Zhang Ying-22455 <ying.zhang22455@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>