BUILD: Clone the Linux tree if it wasn't found via $(LINUX_SRC_PATH)

BUILD: Clone the Linux tree if it wasn't found via $(LINUX_SRC_PATH)
even if it happens to be a local directory.

If we located the Linux tree by deriving the path from "hg path
default" and it is a local path then prepend "file://" to prevent us
trying to symlink it.

Unfortunately mercurial doesn't understand file:// URLs so we have to
strip it off again before calling hg clone.

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