download/git: support Git LFS

Git Large File Storage replaces large files with text pointers in the
Git repository while storing the contents on a remote server.  If a
repository is using this extension, then git-lfs must be used to
checkout the large files before the source archive is generated.

Signed-off-by: John Keeping <john@metanate.com>
[vfazio:
  - add git-lfs to DL_TOOLS_DEPENDENCIES
  - fixup for 5a0d681394
    ("infra/pkg-download: make the DOWNLOAD macro fully parameterised")
]
Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>
[Arnout:
 - don't "git lfs install";
 - recurse into submodules.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

docs/manual/adding-packages-generic.txt

@@ -344,6 +344,10 @@ not and can not work as people would expect it should:
   submodules when they contain bundled libraries, in which case we
   prefer to use those libraries from their own package.
 
+* +LIBFOO_GIT_LFS+ should be set to +YES+ if the Git repository uses
+  Git LFS to store large files out of band.  This is only available for
+  packages downloaded with git (i.e. when +LIBFOO_SITE_METHOD=git+).
+
 * +LIBFOO_STRIP_COMPONENTS+ is the number of leading components
   (directories) that tar must strip from file names on extraction.
   The tarball for most packages has one leading component named