dependencies: check that SSL certificates are installed

Some packages are hosted on https:// servers, and wget only works on
these if the SSL certificates are installed. For example, downloading
the kernel sources from kernel.org requires those SSL certificates to
be installed.

[Peter: fix typo]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>