configs/kontron_smarc_sal28: integrate RCW into rootfs image

Integrate the RCW into the storage device image, so the image can also
be used a boot source. The SoC expects the RCW at offset 4096 of the SD
card or eMMC.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
2 files changed