support/testing: add tests for musl and uclibc toolchains

support/testing: add tests for musl and uclibc toolchains

These tests simply build a system with musl and uclibc toolchains, and
boot them under qemu. It allows to minimally validate that our support
for musl/uclibc external toolchains is working. We already had some
tests covering glibc toolchains, so we can now easily test that all
three C libraries are supported.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
---
This commit is part of the series, as I've written/used those tests to
validate that things are still working correctly with all of glibc,
uclibc and musl toolchains.