pkg-download: Allow packages to pass generic options to download methods

pkg-download: Allow packages to pass generic options to download methods

Introduce a new package variable $(PKG)_DL_OPTS. When this variable
is defined, its value is passed to the downloader as options to
the underlying command. Packages can now retrieve archives from server
expecting logins and passwords, use referer url, proxy or specific
options for cloning a repository.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>