CC1120 preamble set back to 32-bits as there is a strange interraction with 6loWPAN traffic where 16-bit preamble results in reception starting mid-packet (and causing overflows) when used with ContikiMAC.

Other misc improvements/fixes to reduce line count and complexity.
Reverted to single-interrupt mode temporarily.
Merge branch 'rs485-driver2' of https://github.com/feshie/contiki into rs485-driver2