x86: Fix rom version build with CONFIG_X86_RESET_VECTOR

When building U-Boot with CONFIG_X86_RESET_VECTOR, the linking
process misses the resetvec.o and start16.o so it cannot generate
the rom version of U-Boot. The arch/x86/cpu/Makefile is updated to
pull them into the final linking process.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>

arch/x86/cpu/Makefile

@@ -9,5 +9,5 @@
 #
 
 extra-y	= start.o
-extra-$(CONFIG_X86_RESET_VECTOR) += resetvec.o start16.o
-obj-y	= interrupts.o cpu.o
+obj-$(CONFIG_X86_RESET_VECTOR) += resetvec.o start16.o
+obj-y	+= interrupts.o cpu.o