tests: simplify Makefile invocation for tests/tcg

tests: simplify Makefile invocation for tests/tcg

Remove the DOCKER_SCRIPT and TARGET variable from the Makefile invocation
for tests/tcg.  For DOCKER_SCRIPT, resolve the path to docker.py in configure;
for TARGET, move it to config-$(TARGET).mak and use a symbolic link to break
the cycle.

The symbolic link is still needed because tests/tcg includes dummy config files
for targets that are not buildable.  Once that is cleaned up, the symbolic link
will go away too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-20-alex.bennee@linaro.org>