relocate comment to variable declaratiion
Remove serial_cli.c's dependency on mw.h/board.h.

Rename yawdeadband to yaw_deadband to match other variables/cli
commands.

Fix a couple of usages of MAX_MOTORS where MAX_SUPPORTED_MOTORS should
be used instead.
Disambiguate MAX_MOTORS and MAX_SUPPORTED_MOTORS
Achieved by renaming MAX_MOTORS to MAX_PWM_MOTORS

Disambiguate MAX_MOTORS and MAX_SUPPORTED_SERVOS
Achieved by renaming MAX_SERVOS to MAX_PWM_SERVOS

It now shows there is a dependency on the pwm driver if MAX_PWM_* is
used.

Coupled pwm_common and timer_common by using USABLE_TIMER_CHANNEL_COUNT
since the current pwm driver and timer driver is only usable by the
STM32F103.