build system: stubdom targets now depends on mini-os target

Provide mini-os url and revision in Config.mk

Make stubdom targets depend on mini-os-dir target. Make
subtree-force-update{,-all} depend on mini-os-dir-force-update.

Also make mktarball script generate mini-os archive.

Original mini-os directory is renamed to mini-os-intree to help reduce
patch length. That directory will be deleted in a separate patch.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
Changes in v3:
1. Put mini-os targets in main Makefile.
2. Stubdom clean and distclean don't depend on mini-os-dir.

Changes in v2:
1. Use hash in Config.mk.

stubdom/Makefile

@@ -1,6 +1,10 @@
 XEN_ROOT = $(CURDIR)/..
 MINI_OS = $(XEN_ROOT)/extras/mini-os
 
+ifeq ($(wildcard $(MINI_OS)/Config.mk),)
+$(error Please run `make mini-os-dir' in top-level directory)
+endif
+
 export XEN_OS=MiniOS
 
 export stubdom=y