u-boot-mkimage: Fix use of 'cc' instead of BUILD_CC
OE needs to be able to change the default compiler. If we pass in HOSTCC
through the make command, it overwrites not only this setting but also the
setting in tools/Makefile wrapped in ifneq ($(CROSS_BUILD_TOOLS),) which
breaks the build.
We therefore add a way of changing the default in the top level Makefile
without interfering with the other setting.
I've emailed this workaround to Masahiro Yamada for discussion.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
