toolchain: introduce a virtual package

This commit introduces a virtual package called 'toolchain', located
in 'toolchain/toolchain', which simply depends on
'toolchain-buildroot' or 'toolchain-external' depending on the
selected toolchain backend.

For now, toolchain-buildroot and toolchain-external are still manual
make targets, but the following patches convert those backends to use
the package infrastructure as well.

In addition to this:

 * The main Makefile is modified to always make BASE_TARGETS point to
   this new toolchain virtual package.

 * The main Makefile is changed to include all the toolchain/*/*.mk
   files: the toolchain virtual package, and the toolchain-buildroot
   and toolchain-external directories.

 * The dependency of the toolchain on prepare dirs and dependencies is
   moved to the toolchain virtual package. It is moved as a
   prerequisite of the "toolchain-source" rule to ensure that all
   directories are prepared before we even start extracting the
   toolchain.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>