toolchain-external: mark Microblaze external toolchains as deprecated

The Xilinx Microblaze external toolchains that we had support for are
very old, and are causing a huge number of build issues. Thanks to
Spenser Gilliland, we now have support for Microblaze in the internal
toolchain backend, and the autobuilders have been using the internal
toolchain backend since then. Therefore, it's time to deprecate those
old and unusable external toolchains.

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