eglibc: new package

This commit adds a target 'eglibc' package. For now, there is no way
to build this package, the next commit will add the integration of
eglibc into the Buildroot toolchain backend.

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