[Buildroot] [PATCH 0/7] More HOST_DIR/usr fixes

[Arnout Vandecappelle (Essensium/Mind)]

Thomas started rebuilding the toolchains after the HOST_DIR/usr removal
and noticed a corner case that didn't work. While investigating that, I
uncovered a slew of other issues.

I was also in the process of adding a test for
BR2_TOOLCHAIN_EXTERNAL_PREINSTALLED=y but that turns out to require a
lot of refactoring of the testing infra. So it's for another day.

Regards,
Arnout