yocto-bsp: strip '/base' from kernel branches in templates

yocto-bsp: strip '/base' from kernel branches in templates

For new branches, users can specify /base branches, but we don't want
the '/base' in the resultant branch name, so remove it.

Fixes [YOCTO #2693].

(From meta-yocto rev: 40e925862884fd981dec63fc598326c73e4a4c20)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>