docker: drop QEMU_TARGET check, fallback in EXECUTABLE not set

The addition of QEMU_TARGET was intended to ensure we fall back to
checking for the existence of an image if the build system was not
currently configured to build it. However this breaks the direct use
of the rule for building custom binfmt_misc images. We already check
for EXECUTABLE so let us just use that as a proxy for deciding if we
are just going to check the image exits.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/Makefile.include

@@ -58,13 +58,11 @@ docker-image-%: $(DOCKER_FILES_DIR)/%.docker
 docker-binfmt-image-debian-%: $(DOCKER_FILES_DIR)/debian-bootstrap.docker
 	$(if $(EXECUTABLE),,\
 		$(error EXECUTABLE not set, debootstrap of debian-$* would fail))
-	$(if $(wildcard $(EXECUTABLE)),,\
-		$(error Please build $(EXECUTABLE) first))
 	$(if $(DEB_ARCH),,\
 		$(error DEB_ARCH not set, debootstrap of debian-$* would fail))
 	$(if $(DEB_TYPE),,\
 		$(error DEB_TYPE not set, debootstrap of debian-$* would fail))
-	$(if $(filter $(QEMU_TARGET),$(TARGET_DIRS)),				\
+	$(if $(wildcard $(EXECUTABLE)),						\
 		$(call quiet-command,						\
 			DEB_ARCH=$(DEB_ARCH)					\
 			DEB_TYPE=$(DEB_TYPE)					\
@@ -136,7 +134,6 @@ DOCKER_PARTIAL_IMAGES += debian-tricore-cross debian-powerpc-cross fedora-i386-c
 # broken so we need a qemu-linux-user for this target
 docker-binfmt-image-debian-powerpc-user: DEB_ARCH = powerpc
 docker-binfmt-image-debian-powerpc-user: DEB_TYPE = jessie
-docker-binfmt-image-debian-powerpc-user: QEMU_TARGET = ppc-linux-user
 docker-binfmt-image-debian-powerpc-user: EXECUTABLE = ${BUILD_DIR}/ppc-linux-user/qemu-ppc
 docker-image-debian-powerpc-user-cross: docker-binfmt-image-debian-powerpc-user
 DOCKER_USER_IMAGES += debian-powerpc-user