Patch: Add CRC16 to sercomm

Hi everyone,

this patch adds a optional CRC16 checksum to sercomm. To compile it needs the CRC16 in libosmocore.

A bit in the control-byte of the HDLC frame signals if the received frame contains a trailing CRC, if that's the case but the CRC is not ok, the frame is dropped.

Note: For debugging purposes a frame on the ECHO DLCI is not dropped! We might want to check for corruption in the data so we are interested in bad frames!

To send frames with CRC on a specific DLCI use sercomm_tx_set_crc16(int dlci,int onoff); to turn it on. By default CRC16 on Tx is enabled for the ECHO and CONSOLE DLCIs.

Chris