toolchain-external: add new Microblaze toolchains

The existing Microblaze toolchains that we have have the annoying
property of being based on a very old glibc version: 2.3.6. Xilinx
provides newer toolchains with glibc 2.14, generated by Crosstool-NG,
but they are only available as part of a huge Git repository that
contains the gcc, Linux, binutils, glibc sources unpacked (4.4 GB
total), which makes is very unpractical.

I contacted the Xilinx person who did those toolchains, but they
apparently didn't intend to change that anytime soon.

So, we have created a tarball for those toolchains, adding a
README.txt file in the tarball that points back to the original
location that contains the source code for them. Those tarballs are
hosted on sources.buildroot.net.

This commit then adds support for those two new external toolchains,
one for little endian Microblaze, another one for big endian
Microblaze.

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