Add a 'buildtest-indocker' that forces executing 'buildtest' for loop
completely inside the container.
It prevents starting one container per compilation wich is slower but
it could hide errors where the host toolchain would be used
It is currently equivalent to `buildtest` but will change when the
`buidtest` handling will be move outside of `BUILD_IN_DOCKER`.
Display an error when executed without BUILD_IN_DOCKER=1.
When running tests, I often needed to modify `buildtests.inc.mk` to see the
actual errors/standard output.
This allows overwriting the default redirection of both to `/dev/null`.
It is low level and directly given to the command execution.
As it is interpreted by make, it can even be overwritten using build system
variables:
'BUILDTEST_MAKE_REDIRECT=>/tmp/buildtest.$${board}.out 2>&1'
