Beaglebone RS485 cape

To connect with modbus using Beaglebone RS485 cape (http://elinux.org/CircuitCo:CircuitCo/BeagleBone_RS485) there are required a few modifications to the library:
1. You have to make sure that read buffer is empty before writing anything to uart.
2. You have to make sure that write buffer is empty before select function (before reading).
3. You should use one of GPIO ports instead of RTS (due to board design).
Those two commits allow modbus communication using RS485 cape.