add devterm_keyboard_mini init code

Code/devterm_keyboard_mini/math.ino

@@ -0,0 +1,23 @@
+#include <limits>
+
+#include "math.h"
+
+uint32_t getDelta(uint32_t prev, uint32_t now) {
+  uint32_t delta;
+  if (now >= prev) {
+    delta = now - prev;
+  } else {
+    delta = std::numeric_limits<uint32_t>().max() - prev - now + 1;
+  }
+  return delta;
+}
+
+uint32_t getDelta(uint32_t prev, uint32_t now, uint32_t max) {
+  const auto delta = getDelta(prev, now);
+
+  if (delta < max) {
+    return delta;
+  }
+
+  return max;
+}