A while back we had one confused user who apparently had a borked
install due to this. Since it hasn’t been tested, we replace the
original instructions with a help message.

The target may be reintroduced at some point (build TDS, then extract to
local tree), but for now it’s not worth the hassle.

This adds an entry ``loader`` on the ``notes`` hash in the status table
by means of which the fontloader can be specified. The status file is
read when building the configuration table, and if it is present the
entry in the ``loader`` table will be used as the default fontloader.
Otherwise, Luaotfload falls back on the reference loader.

This ensures the file is included in the TDS repository.

During a build process, the name of a freshly created fontloader package
will be passed to ``mkstatus`` so it’ll be considered in the status file.

New targets:

    $ make import

Runs the packaging script in import mode. Also,

    $ make loader

Will package the files in the src/fontloader/* directories into a merged
fontloader package using mtx-package.

As a consequence, imported files not needed at runtime will no longer be
packaged.