60-ukify: kernel-install plugin that calls ukify to create a UKI

60-ukify: kernel-install plugin that calls ukify to create a UKI

60-ukify.install calls ukify with a config file, so singing and policies and
splash will be done through the ukify config file, without 60-ukify.install
knowing anything directly.

In meson.py, the variable for loaderentry.install.in is used just once, let's
drop it. (I guess this approach was copied from kernel_install_in, which is
used in another file.)

The general idea is based on cvlc12's #27119, but now in Python instead of
bash.