docker: par down QEMU_CONFIGURE_OPTS in debian-tricore-cross

This image isn't going to build anything significant as it is just
intended for building test cases. In case it does end up getting
inadvertently included in a build lets aim for the minimal possible
product.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/dockerfiles/debian-tricore-cross.docker

@@ -19,5 +19,5 @@ RUN git clone --single-branch \
     make && make install && \
     rm -rf /usr/src/binutils
 
-# Specify the cross prefix for this image (see tests/docker/common.rc)
-ENV QEMU_CONFIGURE_OPTS --cross-prefix=tricore-
+# This image isn't designed for building QEMU but building tests
+ENV QEMU_CONFIGURE_OPTS --disable-system --disable-user