toolchain/external: newer Linaro toolchains do not provide source code

Currently, we have a pattern-matching that automatically derives the
the source tarball filename from the binary tarball filename.

However, the latest Linaro toolchains no longer follow that scheme (and
do not even readily provide the sources...).

Remove the generic pattern-matching, and explicitly set the source
tarball name for those toolchains that do have a source tarball readily
available.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>