mpc83xx: fix ITX[GP] O=builddir builds

make: *** No rule to make target `/work/wd/tmp/board/mpc8349itx/u-boot.lds', needed by `/work/wd/tmp/u-boot'.  Stop.

Both the ITX and ITX-GP fail when you use "make O=<some dir> ..." or
"BUILD_DIR=<some dir> ./MAKEALL ..."

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