frontend: Add tls-cert and tls-key config options

Add tls-cert and tls-key config options in the [rdp] and [vnc] sections
in weston.ini. This allows to statically configure the TLS key and
certificate files instead of requiring them to be supplied via command
line arguments.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>