The endstop homing system requires all queue_step commands be in the
MCU's 'move queue' before endstop checking starts. Use the normal
message priority system to request that stepper queue_step commands
are received prior to the start of the end_stop_home command. This
simplifies the code and removes the need for special serial queue
flushing.
This also fixes a bug in homing operations that take longer than 2^31
clock ticks.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>