arch-armv7a.inc, tune-arm920t.inc: Fix PACKAGE_EXTRA_ARCHS

* each DEFAULTTUNE with thumb enabled should list it's arm variants in
  PACKAGE_EXTRA_ARCHS, otherwise packages which force arm ISA won't be
  found in do_rootfs
* armv7athf-neon-vfpv4 was missing its own PACKAGE_ARCH and also the arm
  variant

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