The definition of DOCKER_IMAGES and DOCKER_TESTS copes already with an
empty value of $(IMAGES) and $(TESTS), no need to force them to "%" if
undefined.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-5-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-8-alex.bennee@linaro.org>