[chibios] add ADC driver

- added ChibiOS ADC driver for Lisa M 2.0/Lia 1.1 (STM32F105)
- on_chip temperature sensor used for diagnostics
- guard block around adc.h changed (chibiOS uses same header file - adc.h)
- added debug options into chconf.h (slows down CPU, but helps with development)
- lowered ADC interrupt priority so it doesn't slow down CPU

Note:
The ADC driver in ChibiOS uses sequential read only, currently no injected channels.
Lia boards have 4 ADC channels, we are reading all of them continously (circular buffer read),
in the callback the measured values are pushed into the appropriate adc_buf struct (see electrical.c)

Since the conversion runs in the background, it has zero effect on CPU.

Currenly tested only on Lia board, in ChibiOS only ADC1 is defined. Todo is to make driver more general
(i.e. N ADC banks, M channels for each bank).

closes #579
[chibios] add PWM driver

- Added PWM driver for servos, fully compatible with current paparazzi
- Tested for all 8 servos on Lia and Lisa board

closes #580
[chibios] add ppm driver

- Right now ChibiOs supports only first two channels on each timer for time capture
- required modification of ChibiOS (define for Channel 3 for Timers), will push it soon
- uses callbacks for timer updates and event listeners/broadcasting for processing the frame (see the code)
- tested with Turnugy 9xr

Note that once these drivers are finished, I will update the boad config files accordingly. It would just be messy to do it for each driver separately.
The same for main_chibios.c and the threads/events.

closes #581
[chibios] add i2c driver

- added chibios i2c driver interface
- mcu_periph/i2c.c renamed to i2c_pprzi.c because of the filename collision (just the same file with a different name)

closes #590
Merge branch 'rt_paparazzi' of git://github.com/paparazzi/paparazzi into rt_paparazzi