toolchain/crosstool-NG: use the package infrastructure

Move crosstool-NG to package/ and make it an autotarget package.
This requires a new patch (now upstream).

Now, the crosstool-NG toolchain backend only contains enough to build
the toolchain, there's no more reference to building crosstool-NG as
a package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>