support/download: alternative access methods to CVS

Allows user to specify other access methods than :pserver:anonymous@
on CVS repositories. This shall be defined in the <pkg>_SITE variable.

[Thomas:
 - as suggested by Yann, quote the variable expansion
 - as suggested by Yann, use a regexp match
 - tweak commit log]

Signed-off-by: Joao Mano <joao@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>