mx6sabresd: Make checkboard respect CONFIG_NXP_BOARD_REVISION

The default implementation of checkboard() calls the
nxp_board_rev_string() function
to retrieve a character representing the revision number of the board.
However, this
attempt to retrieve the revision number may fail in certain situations or be
otherwise undesirable.

There is already a configuration option to avoid retrieving the
revision number of
the board: CONFIG_NXP_BOARD_REVISION. In fact, if this option is enabled, the
nxp_board_rev_string() function's definition will be omitted entirely,
meaning that the previous implementation of checkboard() would result
in a linker error.

This changeset makes the default implementation of checkboard() respect the
CONFIG_NXP_BOARD_REVISION configuration option, only attempting to retrieve
the board revision number if that option is defined.

Signed-off-by: Cody Gray <cody@codygray.com>