janus-gateway: add HTTP/REST to a new transport config section

janus-gateway: add HTTP/REST to a new transport config section

janus-gateway supports many different transports, and currently there
is no implicit way to turn them off or on. Instead, if the dependency
happens to be built, then the transport is enabled.

Create a transports section in the config file and add
BR2_PACKAGE_JANUS_REST as the first transport.

Signed-off-by: Adam Duskett <aduskett@codeblue.com>
[Thomas: propagate thread dependency.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>