toolchain: include C libraries in legal-info

So far, the legal-info infrastructure was not exporting the legal
info for the C libraries. This is because the legal-info only acts
on packages defined in $(TARGETS).

But the C libraries are never added to $(TARGETS), since there is no
corresponding BR2_PACKAGE_<C-LIBRARY>.

This patch adds such symbols for the 4 C libraries we support in our
internal backend: uClibc, glibc, eglibc and musl.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>