release: Allow powerpc GENERIC64(le)? kernels

The pkgbase-stage.lua script asserts that it has exactly one "kernel",
but only accepts GENERIC as a "kernel". Use a slightly more permissive
regex in order to capture kernels with names which start "GENERIC".

Reviwed by: ivy
MFC after: 12 hours
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52544