tools/libs: create Makefile.common to be used by stubdom build system

This new "Makefile.common" is intended to be used by both tools/ and
stubdom/ build system without stubdom needed to use tools/ build
system.

It should contain the necessary list of objects and CFLAGS needed to
build a static library.

Change stubdom/ to check Makefile.common, for the linkfarm.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

stubdom/Makefile

@@ -365,7 +365,7 @@ define BUILD_lib
 	    $$(MAKE) -C libs-$$(XEN_TARGET_ARCH)/$(1) clean; \
 	fi
 
- libs-$$(XEN_TARGET_ARCH)/$(1)/stamp: $$(XEN_ROOT)/tools/libs/$(1)/Makefile
+ libs-$$(XEN_TARGET_ARCH)/$(1)/stamp: $$(XEN_ROOT)/tools/libs/$(1)/Makefile.common
 	$$(do_links)
 endef