u-boot: add CROSS_COMPILE/ARCH when building host tools

Even when building the host tools, for some reason U-Boot tries to
execute the cross-compiler, so tell it which cross-compiler to use in
order to avoid failure such as:

/usr/bin/make -j12 -C /home/test/outputs/test-253-mini2440_defconfig/build/u-boot-custom tools
make[1]: arm-linux-gcc: Command not found
make[1]: Entering directory `/home/test/outputs/test-253-mini2440_defconfig/build/u-boot-custom'
for dir in tools examples api_examples ; do /usr/bin/make -C $dir _depend ; done
Generating include/autoconf.mk
/bin/sh: line 3: arm-linux-gcc: command not found

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>