[POWERPC] Fix boot wrapper invocation if CROSS_COMPILE contains spaces

My CROSS_COMPILE is "ccache /opt/compilers/blah", which confuses
the boot wrapper script. Quote CROSS_COMPILE and CROSS32_COMPILE
so they can safely contain spaces.
Signed-off-by: NMichael Ellerman <michael@ellerman.id.au>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/boot/Makefile

@@ -105,10 +105,10 @@ wrapperbits	:= $(extra-y) $(addprefix $(obj)/,addnote hack-coff)
 # Bits for building various flavours of zImage
 
 ifneq ($(CROSS32_COMPILE),)
-CROSSWRAP := -C $(CROSS32_COMPILE)
+CROSSWRAP := -C "$(CROSS32_COMPILE)"
 else
 ifneq ($(CROSS_COMPILE),)
-CROSSWRAP := -C $(CROSS_COMPILE)
+CROSSWRAP := -C "$(CROSS_COMPILE)"
 endif
 endif