arm: ensure u-boot only uses relative relocations

Add a Makefile target ('checkarmreloc') which
fails if the ELF binary contains relocation records
of types other than R_ARM_RELATIVE.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Tested-by: Lubomir Popov <lpopov@mm-sol.com>
Tested-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index dc64160..e80e1ed 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -109,3 +109,8 @@
 PLATFORM_RELFLAGS += -fno-optimize-sibling-calls
 endif
 endif
+
+# check that only R_ARM_RELATIVE relocations are generated
+ifneq ($(CONFIG_SPL_BUILD),y)
+ALL-y	+= checkarmreloc
+endif