tests/tcg: move configuration to a sub-shell script

Avoid the repeated inclusions of config-target.mak, which have
risks of namespace pollution, and instead build minimal configuration
files in a configuration script.  The same configuration files can
also be included in Makefile and Makefile.qemu

[AJB 10/09/19]
In the original PR this had inadvertently enabled tests
for ppc64abi32. However as the rest of the multiarch tests work rather
than disabling the otherwise correctly functioning build I've just
skipped the failing linux-test test. For some reason I can't debug it
with TCG so I'm leaving that to the PPC maintainers to look at.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190807143523.15917-4-pbonzini@redhat.com>
[AJB: s/docker/container/, rm last bits from configure, ppc6432abi hack]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Richard Henderson <rth@twiddle.net>