bitbake.conf: Set a dafault value for TUNE_PKGARCH

bitbake.conf: Set a dafault value for TUNE_PKGARCH

If we don't do this, we see an exception:

ERROR: Failure expanding variable MACHINE_ARCH, expression was ${@[d.getVar('TUNE_PKGARCH', True),
d.getVar('MACHINE', True)][bool(d.getVar('MACHINE', True))].replace('-', '_')} which triggered
exception AttributeError: 'NoneType' object has no attribute 'replace'

Setting a default value avoids this error and allows the sanity checker
to trigger instead.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>