uclibc: use uClibc-ng by default

Since the uClibc project is not making any release since about 3
years, and the uClibc-ng project is now alive and delivering regular
releases based on uClibc master plus a number of fixes, let's make
uClibc-ng the default uClibc version used by Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>