xes: Consolidate checkboard()

Create a common checkboard() function to support all X-ES's Freescale
boards.

Also, add a get_board_derivative() function which reads hardware
strapping resistors to determine what model a board is.  This allows one
U-Boot image to support multiple boards.

Signed-off-by: John Schmoller <jschmoller@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>