[POWERPC] bootwrapper: fix build error on virtex405-head.S

virtex405-head.S is an assembler file, not a C file; therefore BOOTAFLAGS
is the correct place to set the needed -mcpu=405 flag.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NJosh Boyer <jwboyer@linux.vnet.ibm.com>

arch/powerpc/boot/Makefile

@@ -40,7 +40,7 @@ $(obj)/ebony.o: BOOTCFLAGS += -mcpu=405
 $(obj)/cuboot-taishan.o: BOOTCFLAGS += -mcpu=405
 $(obj)/cuboot-katmai.o: BOOTCFLAGS += -mcpu=405
 $(obj)/treeboot-walnut.o: BOOTCFLAGS += -mcpu=405
-$(obj)/virtex405-head.o: BOOTCFLAGS += -mcpu=405
+$(obj)/virtex405-head.o: BOOTAFLAGS += -mcpu=405
 
 
 zlib       := inffast.c inflate.c inftrees.c