feature-arm-thumb.inc, arch-armv4.inc: Add "arm" to TUNE_FEATURES

feature-arm-thumb.inc, arch-armv4.inc: Add "arm" to TUNE_FEATURES

* it will be inherited by most DEFAULTTUNEs, except few exceptions which
  support only thumb and not arm
* respect missing "arm" in TUNE_FEATURES in feature-arm-thumb.inc, so
  when recipe asks for "arm" and MACHINE supports only "thumb" ignore
  recipe and try to build with "thumb"
* show warning when overriding ARM_INSTRUCTION_SET set by recipe from tune
  config

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>