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>