linux-yocto: only use intel branches for optimized platforms

linux-yocto: only use intel branches for optimized platforms

Reverting the change that moved common-pc* to the intel staging
branches. This means that genericx86, qemux86*, etc, will continue
to use standard/base.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>