add check infrastructure, default sparse

Add support for running source code checkers on u-boot source, e.g.,
using sparse to aid with typechecking.  This comes in especially
handy as SoC vendors mix and match cores and devices with different
endianness, thus here we add CHECK_ENDIAN to the otherwise linux
kernel default CHECKFLAGS.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>