toolchain-external: conditionalize the installation of libraries

The external toolchain code makes the assumption that all C libraries
have a ld*.so, libc.so, libcrypt.so, libdl.so, libgcc_s.so, libm.so,
libnsl.so, libresolv.so, libutil.so, and when thread support is
enabled, libpthread.so, etc.

However, this is not the case with the musl C library, which
integrates all the functionalities in a single libc.so file. In
preparation of the support of the musl library, we make the current
value of LIB_EXTERNAL_LIBS conditional to glibc or uClibc.

The addition of additional libraries through
BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS is kept outside the condition, at
the end.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>