makefile: add support for *_defconfig targets

Ease up XEN configuration for non-standard builds, like
armv8 tiny config.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>

Makefile

@@ -54,6 +54,10 @@ build: $(TARGS_BUILD)
 build-xen:
 	$(MAKE) -C xen build
 
+.PHONY: %_defconfig
+%_defconfig:
+	$(MAKE) -C xen $@
+
 .PHONY: build-tools
 build-tools: build-tools-public-headers
 	$(MAKE) -C tools build