rename hal-{spi,gpio} to rf230-hal-*
Demo/Shared/Contiki/: provide weak abstraction for GPIO handling
Contiki/rf230bb.h: a few items shared by all rf230bb HALs
Demo/Shared/Contiki/: provide weak abstraction for SPI interface
enable/disable GPIO clock per pin, not at platform level

Summarizing this information in the platform may be more efficient but
also makes the code highly non-modular.