CF/BF - OSD - Show symbol for GPS speed to better align with altitude

display on OSD.  Name and extract the duplicated GPS_speed equation - in
CM/S to KM/H.

src/main/common/maths.h

@@ -35,6 +35,8 @@
 #define DECIDEGREES_TO_RADIANS(angle) ((angle / 10.0f) * 0.0174532925f)
 #define DEGREES_TO_RADIANS(angle) ((angle) * 0.0174532925f)
 
+#define CM_S_TO_KM_H(centimetersPerSecond) (centimetersPerSecond * 36 / 1000)
+
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
 #define MAX(a, b) ((a) > (b) ? (a) : (b))
 #define ABS(x) ((x) > 0 ? (x) : -(x))

src/main/io/osd.c

@@ -220,7 +220,8 @@ static void osdDrawSingleElement(uint8_t item)
 
         case OSD_GPS_SPEED:
         {
-            sprintf(buff, "%d%c", GPS_speed * 36 / 1000, SYM_MS);
+            // FIXME ideally we want to use SYM_KMH symbol but it's not in the font any more, so we use K.
+            sprintf(buff, "%dK", CM_S_TO_KM_H(GPS_speed) * 10);
             break;
         }
 #endif // GPS
@@ -637,7 +638,7 @@ static void osdUpdateStats(void)
 {
     int16_t value = 0;
 #ifdef GPS
-    value = GPS_speed * 36 / 1000;
+    value = CM_S_TO_KM_H(GPS_speed);
 #endif
     if (stats.max_speed < value)
         stats.max_speed = value;