This remove executing buildtest `for` loop in docker.
When building completely in docker, 'buildtest' would hide issues when
the host toolchain would be used when doing `make all` directly.
It has the consequence that it now starts a container for each
compilation which is slower.
The previous behavior can be reproduced by using
BUILD_IN_DOCKER=1 make buildtest-indocker
A side effect is also that now `BUILDTEST_MAKE_REDIRECT` would work when
doing `buildtest` with docker.
7d10da832a