trsync: Introduce new "trigger synchronization" support

Separate out the stepper stopping code from endstop.c into its own
trsync.c code file.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/trsync.h

@@ -0,0 +1,19 @@
+#ifndef __TRSYNC_H
+#define __TRSYNC_H
+
+#include <stdint.h> // uint16_t
+
+struct trsync_signal;
+typedef void (*trsync_callback_t)(struct trsync_signal *tss, uint8_t reason);
+
+struct trsync_signal {
+    struct trsync_signal *next;
+    trsync_callback_t func;
+};
+
+struct trsync *trsync_oid_lookup(uint8_t oid);
+void trsync_do_trigger(struct trsync *ts, uint8_t reason);
+void trsync_add_signal(struct trsync *ts, struct trsync_signal *tss
+                       , trsync_callback_t func);
+
+#endif // trsync.h