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>
4 files changed