missing -lm in drivers/Makefile.am

Both bcmxcp and bcmxcp_usb use ldexp(), so both need `-lm`.

http://news.gmane.org/find-root.php?message_id=544515BA.4060804%40niksoggia.it
docs: docinfo.xml is now auto-generated
riello_ser: enhanced handling error codes (0.03)
riello_usb: timeouts and error handling (0.03)

Small changes in riello_usb.c that solved some problem with managing
transmission errors between the Raspberry Pi and Riello ups (thanks to Fredrik
Öberg): introducing timeout in reading ups data in cypress_command; enhanced
handling error codes

http://news.gmane.org/find-root.php?message_id=f5294e5579574bcbaf5cdd95523e0b68%40AM3PR07MB289.eurprd07.prod.outlook.com