tests/vm: expose BUILD_TARGET, TARGET_LIST and EXTRA_CONFIGURE_OPTS

Now the underlying basevm support passes these along we can expose
some additional variables to our Makefile to allow more customised
tweaking of the build. For example:

  make vm-build-freebsd TARGET_LIST=aarch64-softmmu \
    EXTRA_CONFIGURE_OPTS="--disable-tools --disable-docs" \
    BUILD_TARGET=check-softfloat
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

tests/vm/Makefile.include

@@ -19,6 +19,11 @@ vm-test:
 	@echo ""
 	@echo "  vm-build-all                    - Build QEMU in all VMs"
 	@echo "  vm-clean-all                    - Clean up VM images"
+	@echo
+	@echo "Special variables:"
+	@echo "    BUILD_TARGET=foo		 - override the build target"
+	@echo "    TARGET_LIST=a,b,c    	 - Override target list in builds."
+	@echo '    EXTRA_CONFIGURE_OPTS="..."'
 
 vm-build-all: $(addprefix vm-build-, $(IMAGES))
 
@@ -47,6 +52,9 @@ vm-build-%: $(IMAGES_DIR)/%.img
 		$(if $(J),--jobs $(J)) \
 		$(if $(V),--verbose) \
 		--image "$<" \
-		--build-qemu $(SRC_PATH), \
+		$(if $(BUILD_TARGET),--build-target $(BUILD_TARGET)) \
+		--build-qemu $(SRC_PATH) -- \
+		$(if $(TARGET_LIST),--target-list=$(TARGET_LIST)) \
+		$(if $(EXTRA_CONFIGURE_OPTS),$(EXTRA_CONFIGURE_OPTS)), \
 		"  VM-BUILD $*")