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>
Merge pull request #511 from ADVANSEE/leds

leds: Fixes and enhancements