sanity.bbclass: add more information to error message about TUNE_PKGARCH missing in PACKAGE_ARCHS
* sometimes it's hard to see what's wrong here
* error message:
Error, the PACKAGE_ARCHS variable does not contain TUNE_PKGARCH (cortexa7t2hfhf-vfp-vfpv4-neon).
doesn't help much to understand what exactly went wrong and where
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com> Signed-off-by: Ross Burton <ross.burton@intel.com>