sstate-cache-management.sh: Fix available architectures

sstate-cache-management.sh: Fix available architectures

* grep for AVAILTUNES isn't enough in cases where AVAILTUNE doesn't
  match exactly with TUNE_PKGARCH, e.g. AVAILTUNE "cortexa8thf-neon"
  and TUNE_PKGARCH "cortexa8t2hf-vfp-neon", instead of trying to find
  dynamically every available TUNE_PKGARCH (we have _a lot_ of them
  even with oe-core only), add parameter --extra-archs where user can
  define extra architectures he supports in given build
* Don't replace '-' with '_' for extra-archs, it does apply to MACHINE
  names and some AVAILTUNES, but e.g. cortexa8thf-neon shouldn't be
  converted to cortexa8thf_neon
* Add empty architecture for populate_lic sstate archives
* Add ${build_arch}_${arch} combinations for toolchain recipes (e.g.
  gcc-cross is using x86_64_i586

(From OE-Core rev: a27cc54fb2d0e59f3a800893c1848cb26a7c5335)

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