avr: pgm: added a program memory library

The library functions/definitions in <asm/pgm.h> can be used to read
data from program memory on HARVARD architectures.

Signed-off-by: Michel Megens <dev@michelmegens.net>
gpio: atmega: moved pin settings to progmem.

* Pin configurations are stored in program memory from now on.

platform:
* Added platform driver files.

Signed-off-by: Michel Megens <dev@michelmegens.net>
platform: uno: implemented pgpio for the UNO.

Implemented a platform GPIO driver for the Arduino UNO.

Signed-off-by: Michel Megens <dev@michelmegens.net>
platform: uno: doc: added doxygen comments

Added doxygen comments to these API functions:

	* pgpio_read_pin(int pin
	* pgpio_direction_output
	* pgpio_direction_input
	* pgpio_pin_release
	* pgpio_pin_request
	* pgpio_write_pin

Signed-off-by: Michel Megens <dev@michelmegens.net>