Deprecate the support for the toolchain on target

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>