toolchain: add support for glibc

The support for eglibc 2.17 was added to the internal toolchain
backend for 2013.08. This commit now adds glibc 2.18 support to the
internal toolchain backend.

Since the building procedure is very similar to the one of eglibc, we
have renamed the 'eglibc' package to 'glibc', and made it capable of
handling either glibc or eglibc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>