board: sl28: implement additional bootsources

The board is able to boot from the following source:
 - user-updateble SPI flash
 - write-protected part of the same SPI flash
 - eMMC
 - SD card

Implement the needed function hooks to support all of these boot
sources.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
5 files changed