pkgbase: set a default set of kernel for when PACKAGE_BUILDING=1

PACKAGE_BUILDING is already known in the ports tree as a variable
use to defined when the packages is being actually built in an
automation process, reuse that variable to define the default set
of kernel we plan to build for the default pkgbase.

Reviewed by: manu, jrtc27
Accepted by: manu
MFC After: 1 week
Differential Revision: https://reviews.freebsd.org/D42444

(cherry picked from commit df6e7e11c5f660cbd5381819718e3c947325fa2e)