pkgbase: add a mechanism to be able to force a give ucl include

This is made in order to be able to find add the post-install scripts
for the kernel, where PKGNAME varies for each KERNCONF but we don't want
to dynamically duplicated the kernel.ucl file.

At the same time we don't want the *-dbg* packages to actually include
those post-install scripts

(cherry picked from commit 45d83c39262d9e2727c6f409e19c653f19a5d7cc)