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>
diff --git a/board/kontron/sl-mx8mm/sl-mx8mm.c b/board/kontron/sl-mx8mm/sl-mx8mm.c
index 416c4cb..6e73ede 100644
--- a/board/kontron/sl-mx8mm/sl-mx8mm.c
+++ b/board/kontron/sl-mx8mm/sl-mx8mm.c
@@ -121,6 +121,12 @@
 	return 0;
 }
 
+int board_late_init(void)
+{
+	env_set("som_type", "sl");
+	return 0;
+}
+
 enum env_location env_get_location(enum env_operation op, int prio)
 {
 	enum boot_device boot_dev = get_boot_device();