kontron-sl-mx8mm: fix missing configs and deadlock in CI

Even if board can be successfuly built, CI goes in deadlock (see thread
on https://www.mail-archive.com/u-boot@lists.denx.de/msg419663.html

).
This is caused by SYS_CONFIG set in header file and because defconfig
for the board is out of sync with Kconfig. As result, buildman goes on
to read from stdin until an OOM is reached.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Frieder Schrempf <frieder.schrempf@kontron.de>