kernel-install: add "$KERNEL_INSTALL_STAGING_AREA" directory

kernel-install: add "$KERNEL_INSTALL_STAGING_AREA" directory

The general approach of kernel-install was that each plugin would drop in some
files into the entry directory. But this doesn't scale well, because if we have
multiple initrd generators, or multiple initrds, each generator would need to
recreate the logic to put the generated files in the right place.

Also, effective cleanup is impossible if anything goes wrong on the way, so we
could end up with unused files in $BOOT.

So let's invert the process: plugins drop files into $KERNEL_INSTALL_STAGING_AREA,
and at the end 90-loaderentry.install DTRT with those files.

This allow new plugins like 50-mkosi-initrd.install to be significantly simpler.