feature-arm-thumb.inc: Fix ARMPKGSFX_THUMB value

* my previous thumb related commit:
  commit 3e760031
  Author: Martin Jansa <martin.jansa@gmail.com>
  Date:   Wed Feb 18 15:40:35 2015 +0100

    feature-arm-thumb.inc: respect ARM_INSTRUCTION_SET when adding thumb
    suffix

  unfortunately removed conditional on "thumb" in TUNE_FEATURES, when
  setting ARMPKGSFX_THUMB

* in case we have MACHINE without "thumb" in TUNE_FEATURES and distro
  setting ARM_INSTRUCTION_SET to "thumb" we end with:
  ARM_INSTRUCTION_SET="thumb"
  ARM_THUMB_OPT="thumb"
  ARM_M_OPT="thumb"

  # TUNE_CCARGS correctly not adding -mthumb
  TUNE_CCARGS=" -march=armv7-a  -mthumb-interwork -mfloat-abi=softfp -mfpu=neon"

  # but ARMPKGSFX_THUMB and TUNE_PKGARCH including "t2":
  ARMPKGSFX_THUMB="t2"
  TUNE_PKGARCH="armv7at2-vfp-neon"

  # causing following error:
  Error, the PACKAGE_ARCHS variable does not contain TUNE_PKGARCH (armv7at2-vfp-neon).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>