Deprecate the support for the toolchain on target

As discussed during the ELCE 2012 Buildroot Developers Meeting, we no
longer want to support the possibility of building a toolchain for the
target. None of the core developers have any use for this, it has been
known to be broken or cause problems for a long time without anyone
providing fixes for it.

In addition to this, Buildroot is inherently a cross-compilation tool,
so the usage of a native toolchain on the target is not really
useful. Many newcomers are tempted to use this possibility even though
it is clearly not the intended usage of Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>