port 78d634e8dc84b3c11611480000357efeea0b0027 from Pd-l2ork: fixed regression as per Albert's request

*added new externals for raspberry pi (disis_gpio for digital I/O and disis_spi for analog I/O using mcp3008 A/D converter). Provided preliminary help files. PWM on gpio needs to be tested further. So does the build script. TODO: add this to the installer, including 4755 chown_gpio script.