support/download/cvs: add support to use a date instead of a tag

This is useful when a tag is not avaiable.

Also fix support for Fedora where the command "cvs -r :<version>" doesn't work.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>