imx: kontron-sl-mx8mm: Prepare for other i.MX8MM SoM types

This sets an env variable 'som_type' from the board code. It can
later be used by environment scripts, e. g. to select the proper
devicetree for the board.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
2 files changed