build.include: add support for git:// uris

git:// uris can be declared in .spec files as follows:

[{cross,pkg}=<target>]
...
url = git://<host+stuff>/<repo name>.git[@<refname>] <tarball file name + ext>
...

The tarball file name is important because it makes possible to create a
tarball archive out of the git checkout, which is good for both the ubuild build
cache and sources_dir
specs: add u-boot target
specs: move specs/ out of src/