support/scripts/gen-bootlin-toolchains: add new script to support Bootlin toolchains

support/scripts/gen-bootlin-toolchains: add new script to support Bootlin toolchains

https://toolchains.bootlin.com/ has been providing for a few years a
number of ready-to-use pre-built toolchains, for a wide range of
architectures (which it turns out, are all built using Buildroot).

While toolchains.bootlin.com provides Buildroot config fragments to
easily use those toolchains with Buildroot (see [0] for example), this
is not visible anywhere. So instead, we would like to add support for
these toolchains in Buildroot just like we have existing support for
Linaro, ARM, Synopsys, etc. toolchains.

[0] https://toolchains.bootlin.com/downloads/releases/toolchains/aarch64/fragments/aarch64--glibc--bleeding-edge-2020.02-2.frag

However, the number of toolchains provided by toolchains.bootlin.com
is really large, and they are regularly updated. Maintaining that
manually would be time consuming and error-prone. So instead, this
commit introduces a script that automatically generates:

 - toolchain/toolchain-external/toolchain-external-bootlin/Config.in.options
 - toolchain/toolchain-external/toolchain-external-bootlin/toolchain-external-bootlin.mk
 - toolchain/toolchain-external/toolchain-external-bootlin/toolchain-external-bootlin.hash
 - support/testing/tests/toolchain/test_external_bootlin.py

We create a single external toolchain package, with a Kconfig "choice"
as a sub-option to select the toolchain variant to be used. The script
contains a Python dict that provides the mapping between the
toolchains provided by toolchains.bootlin.com, and the architecture
options/variants they are applicable to.

The test cases allow to verify that the toolchain configuration is
correct, and that it is able to build a Busybox based system. It
doesn't do any runtime testing as such testing is already done by
toolchains.bootlin.com: the test cases here are only meant to verify
that the toolchain-external-bootlin package works as expected.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Titouan Christophe <titouan.christophe@railnova.eu>
Tested-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>