Since "333ff7b1e4 Fix linking with new ld, based on u-boot" the build
with multiple images became unreasonable slow. This is because the
ld-option macro was evaluated once for each image. Fix this by exporting
and using LDFLAGS_barebox from the main Makefile.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>