x86/boot: Use CROSS_COMPILE prefix for readelf

The boot code Makefile contains a straight 'readelf' invocation. This
causes build warnings in cross compile environments, when there is no
unprefixed readelf accessible via $PATH.

Add the missing $(CROSS_COMPILE) prefix.

[ tglx: Rewrote changelog ]

Fixes: 98f7852537 ("x86/boot: Refuse to build with data relocations")
Signed-off-by: Rob Landley <rob@landley.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: "H.J. Lu" <hjl.tools@gmail.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/ced18878-693a-9576-a024-113ef39a22c0@landley.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/boot/compressed/Makefile

@@ -94,7 +94,7 @@ vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
 quiet_cmd_check_data_rel = DATAREL $@
 define cmd_check_data_rel
 	for obj in $(filter %.o,$^); do \
-		readelf -S $$obj | grep -qF .rel.local && { \
+		${CROSS_COMPILE}readelf -S $$obj | grep -qF .rel.local && { \
 			echo "error: $$obj has data relocations!" >&2; \
 			exit 1; \
 		} || true; \