toolchain/crostool-NG: mark as deprecated

For the following reasons:
  - it used to be broken without anyone noticing for a long time,
  - it is still not fully integrated within the Buildroot set of options,
  - it has not gained much traction (not even I use it),
  - I've always argued that sustained development should use an external
    toolchain, and not rely on building one with Buildroot,
  - I did not submit any of the enhancements requested during the last
    developpers' day in Brussels,
  - I have neither the incentive nor the time to maintain and enhance it,

it is time to deprecate the crosstool-NG backend for the 2013.05 release.

Then, it will be entirely removed early in the 2013.08 cycle, to let some
time for those that rely on it to voice their opinions. ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>