arm/arch-arm*: define ARMPKGARCH_tune-* for default tunes

arm/arch-arm*: define ARMPKGARCH_tune-* for default tunes

* tune-foo is not valid override, for it to work I had to add
  ARMPKGARCH = "${ARMPKGARCH_tune-${DEFAULTTUNE}}"
  but that doesn't work without value defined for every supported
  DEFAULTTUNE value, otherwise it's expanded like this
  TUNE_PKGARCH (${ARMPKGARCH_tune-armv5te}te).

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