board: sl28: add config to enable console output on SER0

Sometimes it is desireable to have the console output on the first
serial line. Introduce a configuration option for it (in the board
scope).

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/board/kontron/sl28/Makefile b/board/kontron/sl28/Makefile
index 147ef98..5d220f0 100644
--- a/board/kontron/sl28/Makefile
+++ b/board/kontron/sl28/Makefile
@@ -4,7 +4,7 @@
 obj-y += sl28.o cmds.o
 endif
 
-obj-y += ddr.o
+obj-y += common.o ddr.o
 
 ifdef CONFIG_SPL_BUILD
 obj-y += spl.o