Makefile: Add a 'checkthumb' rule

This rule confirms that if we're on ARM and we have enabled THUMB builds
that we have a new enough toolchain to produce a working binary.

Changes in v2:
- Switch to ALL-$(CONFIG_SYS_THUMB_BUILD) in arch/arm/config.mk (Mike F)
- Simplfy checkthumb test after doing the above

Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/Makefile b/Makefile
index 023ea23..351a8f0 100644
--- a/Makefile
+++ b/Makefile
@@ -556,6 +556,13 @@
 $(obj)System.map:	$(obj)u-boot
 		@$(call SYSTEM_MAP,$<) > $(obj)System.map
 
+checkthumb:
+	@if test $(call cc-version) -lt 0404; then \
+		echo -n '*** Your GCC does not produce working '; \
+		echo 'binaries in THUMB mode.'; \
+		echo '*** Your board is configured for THUMB mode.'; \
+		false; \
+	fi
 #
 # Auto-generate the autoconf.mk file (which is included by all makefiles)
 #