release: Improve kernel package handling

Although support for PowerPC kernels (GENERIC64 and GENERIC64LE) was
added to pkgbase-stage.lua, the equivalent support was missing from
bsdinstall, so the installer would fail at runtime since it wouldn't
find a kernel package.

Improve the pkgbase-stage logic to have a specific list of kernels
we want to support (which avoids breaking if multiple kernels are
available), and use the same logic in both bsdinstall and pkgbase-stage.

MFC after: 1 day
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D52638