config: Always use GNU ld

This patch makes sure that we always use the GNU ld. U-Boot uses certain
construct e.g. OVERLAY which are not implemented in gold therefore it
always needs GNU ld for linking.

It works well if default linker in toolchain is GNU ld but in some
cases we can have gold to be the default linker and also ship GNU ld
but not as default in such cases its called $(PREFIX)ld.bfd, with this
patch we make sure that if $(PREFIX)ld.bfd exists than we use that for
our ld.

This way it does not matter what the default ld is.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>