Re-implement the source-check and external-deps targets

The new DL_MODE variable dispatches between the various download
implementations of each method (Git, Subversion, Wget) to deal with the
normal download (default mode, 'DOWNLOAD'), the source-check
('SOURCE_CHECK') and to show the external dependencies for external-deps
('SHOW_EXTERNAL_DEPS').

For the latter, the legacy script wget-show-external-deps.sh is no
longer required as $(WGET) isn't called directly anymore but always
through the DOWNLOAD helper.

Signed-off-by: Maxime Petazzoni <maxime.petazzoni@bulix.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>