make: Check tools/qemu-xen[-traditional] for qemu before downloading

Currently xen, qemu-xen, and qemu-xen-traditional are kept in separate
repositories, but when we release them as a tarball, qemu-xen and
qemu-xen-traditional are in-lined into the tools/ directory.

In order to make this "just work", at the moment developer doing the
release manually modifies Config.mk as part of the relase process so
that CONFIG_QEMU and QEMU_UPSTREAM_URL point into the tools/ directory
instead.

Modify Config.mk to automatically check there before trying a remote
repository.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

Config.mk

@@ -224,6 +224,21 @@ else
 QEMU_REMOTE ?= git://xenbits.xen.org/qemu-xen-unstable.git
 endif
 
+# Where to look for inlined subtrees (for example, from a tarball)
+QEMU_UPSTREAM_INTREE ?= $(XEN_ROOT)/tools/qemu-xen
+QEMU_TRADITIONAL_INTREE ?= $(XEN_ROOT)/tools/qemu-xen-traditional
+
+
+# Specify which qemu-dm to use. This may be `ioemu' to use the old
+# Mercurial in-tree version, or a local directory, or a git URL.
+# CONFIG_QEMU ?= `pwd`/$(XEN_ROOT)/../qemu-xen.git
+CONFIG_QEMU ?= $(or $(wildcard $(QEMU_TRADITIONAL_INTREE)),\
+                    $(QEMU_REMOTE))
+
+ifneq (,$(wildcard $(QEMU_UPSTREAM_INTREE)))
+QEMU_UPSTREAM_URL ?= $(QEMU_UPSTREAM_INTREE)
+endif
+
 ifeq ($(GIT_HTTP),y)
 OVMF_UPSTREAM_URL ?= http://xenbits.xen.org/git-http/ovmf.git
 QEMU_UPSTREAM_URL ?= http://xenbits.xen.org/git-http/qemu-upstream-unstable.git
@@ -241,10 +256,6 @@ SEABIOS_UPSTREAM_TAG ?= rel-1.7.5
 
 ETHERBOOT_NICS ?= rtl8139 8086100e
 
-# Specify which qemu-dm to use. This may be `ioemu' to use the old
-# Mercurial in-tree version, or a local directory, or a git URL.
-# CONFIG_QEMU ?= `pwd`/$(XEN_ROOT)/../qemu-xen.git
-CONFIG_QEMU ?= $(QEMU_REMOTE)
 
 QEMU_TAG ?= d0395cc49b2ec6d1723c01f1daf2394b9264ca29
 # Tue Apr 8 16:50:06 2014 +0000