package/mg: new package

package/mg: new package

Mg is a small Emacs-like editor with no external dependencies except a
standard C library.  It weighs in at 130 kiB and is one of a select few
completely free (public domain) text editors suitable for small and
embedded systems.

This version is based on the OpenBSD Mg, but with more features, one of
which being the no-ncurses/termcap support, which heavily reduces the
impact on a resource constrained system.

Upstream: https://github.com/troglobit/mg/

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>