feature-arm-thumb.inc: Suppress false warning

If a recipe does not explicitly set ARM_INSTRUCTION_SET, then there is no
need to throw a warning:

  WARNING: Recipe 'foobar' selects ARM_INSTRUCTION_SET to be 'None',
           but tune configuration overrides it to 'arm'

Signed-off-by: Jacob Kroon <jacob.kroon@mikrodidakt.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
feature-arm-thumb.inc: set ARMPKGSFX_THUMB only when thumb is in TUNE_FEATURES

* there is issue for TUNE_PKGARCH missing in PACKAGE_ARCHS for machines
  without thumb enabled, it was reported by Jacob Kroon on IRC

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>