fiq_debugger: Add fiq_debugger.disable option

This change allows to use same kernel image with
different console options for uart and fiq_debugger.
If fiq_debugger.disable will be set to 1/y/Y,
fiq_debugger will not be initialized.

Change-Id: I71fda54f5f863d13b1437b1f909e52dd375d002d
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>