external-toolchain: BR2_TOOLCHAIN_EXTERNAL_GLIBC includes eglibc

Yann E. MORIN says:

    "Although eglibc can be configured to include/exclude parts of the
    features, it seems to not be in wide use, if at all."

Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>