package/freerdp: Always install key and certificate

Currently, the keys are only installed if the server is enabled.

However, other packages (e.g. weston) may implement an RDP server,
using the FreeRDP library.

So, we must always install the key and certificate.

Install them world-readable so non-root users may start an RDP server
without requiring to generate their own keys.

Add a comment in the help text about key and certificate management.

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