board: sl28: add board specific nvm command

The board supports 16 configuration bits which can be manipulated with
this command. See the board's README for a detailed explanation on each
bit.

Signed-off-by: Michael Walle <michael@walle.cc>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>