barebox: fix compilation on x86-64 architecture

Barebox uses KERNEL_ARCH to derive BAREBOX_ARCH, but doesn't
currently handle the case of the x86-64 architecture. In this
case KERNEL_ARCH is x86_64, but BAREBOX_ARCH should be x86
nonetheless.

Signed-off-by: Fabien Lahoudere <fabienlahoudere.pro@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/barebox/barebox.mk

@@ -41,6 +41,8 @@ endif
 
 ifeq ($(KERNEL_ARCH),i386)
 BAREBOX_ARCH = x86
+else ifeq ($(KERNEL_ARCH),x86_64)
+BAREBOX_ARCH = x86
 else ifeq ($(KERNEL_ARCH),powerpc)
 BAREBOX_ARCH = ppc
 else