uclibc: enable debug build option

Set uClibc DODEBUG in case BR2_ENABLE_DEBUG is set.

Setting DODEBUG was removed with commit 70b7079d
in Dec. 2013 because of build failures with the old uClibc (now replaced
with uClibc-ng), no build failure with raspberrypi3_defconfig and
BR2_ENABLE_DEBUG enabled.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>