arm: armv7m: Clean up some thumb / compiler flag options

- The correct way to build with thumb mode is to select SYS_THUMB_BUILD
- We should be setting -march=armv7-m in arch/arm/Makefile not the
  sub-config.mk file.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 4d6d276..680c6e8 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -16,6 +16,7 @@
 arch-$(CONFIG_CPU_ARM1176)	=-march=armv5t
 arch-$(CONFIG_CPU_V7A)		=$(call cc-option, -march=armv7-a, \
 				 $(call cc-option, -march=armv7, -march=armv5))
+arch-$(CONFIG_CPU_V7M)		=-march=armv7-m
 arch-$(CONFIG_CPU_V7R)		=-march=armv7-r
 arch-$(CONFIG_ARM64)		=-march=armv8-a