Updates for XEN_LINUX_SOURCE=hg-clone.

Updates for XEN_LINUX_SOURCE=hg-clone.

By default search for mercurial repositories in $(LINUX_SRC_PATH)
followed by a location derived from the current Xen repository's
default parent. e.g. A clone of
http://xenbits.xensource.com/xen-unstable.hg will derive
http://xenbits.xensource.com/linux-2.6.18-xen.hg. A specific
repository can be specified by giving XEN_LINUX_HGREPO=<URL or path>
on the make command line.

Add capability to put XEN_LINUX_UPDATE=y on the make command line to
update the Linux repository. Disabled by default since we don't want
to touch a developers repository without permission.

Dropped XEN_LINUX_HGUPDATES since it was unecessary and confusing.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>