Make it a choice menu which offers the 3 C libraries we know about: glibc,
uClibc and musl. While at it, make it possible for the external toolchain libc
to select USE_GLIBC, USE_UCLIBC or USE_MUSL which is used by several packages
to conditionally include specific CFLAGS (e.g: iproute2).

Because USE_GLIBC et al. can now be selected by external toolchains, we need to
restrict the per-libc menus to check on !EXTERNAL_TOOLCHAIN.

While at it, make musl the default C library for external toolchain to match
the internal toolchain.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>