Makefile: add LPC32xx precondition for building platform images

To prevent accidental build failures the change converts a number of
NXP LPC32xx specific image targets to be conditionally dependent on
target build configuration. The wrapped image targets always contain
a U-Boot SPL binary and the images are supposed to be directly flashed
on a NAND flash device for read access by LPC32xx NAND MLC controller.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>