toolchain: add a hidden config option to enable the toolchain package

The usual way to enable a package using the package infrastructure is to
use a config option so instead to add the toolchain package to the
TARGETS variable in the Makefile add a config option like all the other
toolchain packages.

[Thomas: remove comment that no longer made sense in the main
Makefile, and add a comment above the new hidden Config.in option to
explain what it is useful for.]

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>