u-boot: Ensure we use BFD as linker even if using GCC for it

u-boot: Ensure we use BFD as linker even if using GCC for it

If we are using 'ld-is-gold' as DISTRO_FEATURE, the U-Boot Makefile has
mechanisms to ensure that we use BFD as linker. However, this does not
work for the GCC when it is used as linking tool.

To support that, we added explicit BFD use whenever we have the
DISTRO_FEATURE feature enabled.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>