bluez5: add "daemon" mode for l2test

This MR is a first part of fix for [YT-193] EMC Bluetooth test: l2test based.

Reworked l2test to fulfill requirements articulated by SECO:

• added a daemon mode. In this mode, l2test will establish connection to the remote server and wait for SIGUSR1 signal from user to start transfer of data over Bluetooth;
• this new mode is compatible with the most of existing arguments, including -b bytes — the size of BT packages which are transferred during test.

The second part of fix is provided in following MR: seco-ne/tools/gf-emc-test-suite!261.