package/freerdp: install server key and certificate

This is mandatory for an RDP server to have a key and a certificate,
otherwise clients will refuse to connect to that server.

We install the key and certificate bundled in FreeRDP. The user can
install its own set using a post-build script if needed.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>