tests: add top-level make dependency for docker builds

One problem with satisfying your docker dependencies in a sub-make it
you might end up trying to satisfy the dependency multiple times. This
is especially a problem with debian-sid based cross compilers and CI
setups. We solve this by doing a docker build pass at the top level
before any sub-makes are called.

We still need to satisfy dependencies in the Makefile.target call so
people can run tests from individual target directories. We introduce
a new Makefile.probe which gets called for each PROBE_TARGET and
allows us to build up the list. It does require multiply including
config-target.mak which shouldn't cause any issues as it shouldn't
define anything that clashes with config-host.mak. However we undefine
a few key variables each time around.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

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