conf/machine: use .= instead of += in TUNE_CCARGS

* number of TUNE_CCARGS conditionals is important if we add
  extra space with each one in "else" branch

  I'm building for 2 MACHINEs one is cortexa9, second is cortexa8

  few months ago we added TUNE_CCARGS[vardepvalue] in bitbake.conf
  http://git.openembedded.org/openembedded-core/commit/?id=03f1e34ea3ce80931e9c3cd2ab22824f28a7233b

  which fixed some cases (like mentioned tune-xscale and tune-arm926ejs)
  where both had unused TUNE_CCARGS when common DEFAULTTUNE was used.

  with cortexa[89] it's different, because cortexa9 has one extra TUNE_CCARGS
  TUNE_CCARGS += "${@bb.utils.contains("TUNE_FEATURES", "cortexa9", "-mtune=cortex-a9", "", d)}"
  which adds extra *space* even when not used because of '+=' and as result:

  $ bitbake-diffsigs tmp-eglibc/sstate-diff/1366797730/*/armv7*/adapterbase/*do_configure*
  basehash changed from f986789fb8fb3579ed6a3492cc8a8d10 to c851b5f838d945ee13072e9ad6725dca
  Variable TUNE_CCARGS value changed from
  ' -march=armv7-a     -mthumb-interwork -mfloat-abi=softfp -mfpu=neon '
  to
  ' -march=armv7-a     -mthumb-interwork -mfloat-abi=softfp -mfpu=neon  '
  Hash for dependent task gcc-runtime_4.7.bb.do_populate_sysroot changed from bdeabf7a86958b9110b566344b7916de to 2be5618e6bc8c57ec9db5659bf217915
  Hash for dependent task eglibc_2.17.bb.do_populate_sysroot changed from b4f40fc62dde684acd0a574532a55360 to 97fcb426603d4a1c1099c0504d2ebf7d
  Hash for dependent task glib-2.0_2.34.3.bb.do_populate_sysroot changed from fd2f90b83098c34e88d649d70f6ea4f5 to ebd740bb94ea3eb0a914efda6fc82c4a

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
tune-thumb.inc: Remove, replaced by arm/feature-arm-thumb.inc

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>