leds: Fix the API

The leds API did not work in some cases. E.g. with the following sequence:
  leds_off(LEDS_ALL);
  leds_toggle(LEDS_GREEN);
  leds_off(LEDS_ALL);
the green LED was remaining on after the last call.

This was caused by the toggle feature made synonymous with the invert feature,
although it is unrelated. leds_toggle() is indeed supposed to toggle an LED,
while leds_invert() is supposed to change the active level of an LED. However,
all users of leds_invert() actually meant leds_toggle(), and the invert feature
does not make sense in this module because it is not handy due to successive
calls to leds_invert() changing the intended behavior, and hardware active
levels should be managed in leds_arch_set() (e.g. by XORing the passed value
with a hardware-specific constant before setting the output levels of the pins).

Consequently, this change:
 - removes the leds_invert() function,
 - makes leds_toggle() behave as expected relatively to leds_off() / leds_on(),
 - sanitizes the code in the leds module.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
leds: Add the leds_set() function

The leds_set() function is added on top of leds_arch_set() in order to have a
means of displaying a pattern on a set of LEDs, while keeping the ENERGEST
information up to date, which would be missing with a direct call to
leds_arch_set().

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Redefine assert if already defined since some platforms already have an assert definition
Use platform configuration for CC2420/CC2520 SFD on MSP430 based platforms
Fixed typo to correctly print the radio channel at startup
CONTIKI_PROJECT is sometimes set to multiple files which caused distclean to fail.
Extended radio API definitions with support for setting channel, pan id, etc.
Added extended radio API implementation for STM32W
Added stub for extended radio API for CC2420
Added stub for extended radio API for CC2520
Added stub for extended radio API for nullradio
Added stub for extended radio API for rf230
Added stub for extended radio API for cc2430
Added stub for extended radio API for cc2538
Added stub for extended radio API for cc253x
Added stub for extended radio API for mc1322x
Added stub for extended radio API for rl78
Added stub for extended radio API for Cooja
Added stub for extended radio API for seedeye
Enhanced the documentation.