motion: drive joint.N.index-enable only on edges

joint.N.index-enable is a HAL_IO pin: homing writes 1 to arm an index
search, the encoder driver writes 0 to clear it on the index pulse.
base_write_homing_out_pins wrote it every servo cycle, clamping a pin
homing does not exclusively own and holding it low after homing finished.

That fights any other writer on the signal. Netting it to
spindle.N.index-enable breaks G33/G33.1: idle homing overwrites the
spindle's sync request low and the move rapids instead of waiting for
the index.

Drive the pin only on homing's own edges (arm on rising, release on
falling) and otherwise leave it for the encoder to clear.

Author: grandixximo

src/emc/motion/homing.c

@@ -111,6 +111,7 @@ typedef struct {
   bool         homed;                // OUT pin
   bool         home_sw;              // IN  pin
   bool         index_enable;         // IO  pin
+  bool         index_enable_prev;    // last value driven onto index_enable pin
   bool         joint_in_sequence;
   int          pause_timer;
   double       home_offset;          // intfc, updateable
@@ -559,7 +560,16 @@ static void base_write_homing_out_pins(int njoints)
         *(addr->homing)       = H[jno].homing;       // OUT
         *(addr->homed)        = H[jno].homed;        // OUT
         *(addr->home_state)   = H[jno].home_state;   // OUT
-        *(addr->index_enable) = H[jno].index_enable; // IO
+        // index_enable is a HAL_IO pin: the encoder driver also writes it
+        // (clears it on the index pulse). We do not own it, so we must not
+        // clamp it every cycle; drive it only on our own edges and otherwise
+        // leave it to whoever else is on the signal.
+        if (H[jno].index_enable && !H[jno].index_enable_prev) {
+            *(addr->index_enable) = 1; // arm the index search
+        } else if (!H[jno].index_enable && H[jno].index_enable_prev) {
+            *(addr->index_enable) = 0; // release if aborted while armed
+        }
+        H[jno].index_enable_prev = H[jno].index_enable;
     }
 }