toolchain/toolchain-buildroot: migrate to virtual package infrastructure

This patch migrates the toolchain and toolchain-buildroot packages to the
virtual package infrastructure, causing the log messages to change from:

>>> toolchain undefined Downloading
>>> toolchain undefined Extracting
...

to

>>> toolchain virtual Downloading
>>> toolchain virtual Extracting
...

and similar for 'toolchain-buildroot', simply because it looks nicer.

At the same time, the directory names also become toolchain-virtual,
toolchain-buildroot-virtual instead of the corresponding 'undefined'
variants.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>