uclibc: do not enable DODEBUG

The DODEBUG configuration option in uClibc is broken, even for widely
used architectures like ARM. Therefore, any Buildroot configuration
that has BR2_ENABLE_DEBUG enabled and uses uClibc with the internal
toolchain backend will fail, with an error like:

  http://autobuild.buildroot.net/results/d34/d34238c209cf2ef63371a598ab9218165c90f864/build-end.log



This error was also reported recently by a Buildroot user.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>