prserv: use only PRSERV_HOST

- remove PRSERV_PORT variable
- use 'hostname:port' as PRSERV_HOST format
- use 'localhost:0' for enabling the local PRServer

[YOCTO #3744]

Signed-off-by: Constantin Musca <constantinx.musca@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>