afboot-stm32: use the Buildroot toolchain

afboot-stm32: use the Buildroot toolchain

By default, the afboot-stm32 Makefile uses "CROSS_COMPILE =
arm-none-eabi-". Since I had such a toolchain installed on my system
when testing afboot-stm32, I didn't realize it wasn't using the
Buildroot toolchain.

However, using the Buildroot toolchain doesn't immediately works for
FLAT toolchains, as gcc automatically wants to create a FLAT
binary. So we need to adjust the afboot-stm32 Makefile to use directly
'ld' and not 'gcc' when linking.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>