Makefile: fix readelf usage

Some OS (like OS X) do not provide a generic readelf. We should enforce to use
the toochain provided readelf instead, to do so use $(CROSS_COMPILE)readelf.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Tested-by: Lubomir Popov <lpopov@mm-sol.com>

Makefile

@@ -747,7 +747,7 @@ endif	# config.mk
 # ARM relocations should all be R_ARM_RELATIVE.
 checkarmreloc: $(obj)u-boot
 	@if test "R_ARM_RELATIVE" != \
-		"`readelf -r $< | cut -d ' ' -f 4 | grep R_ARM | sort -u`"; \
+		"`$(CROSS_COMPILE)readelf -r $< | cut -d ' ' -f 4 | grep R_ARM | sort -u`"; \
 		then echo "$< contains relocations other than \
 		R_ARM_RELATIVE"; false; fi