Respect feature AIRMODE

While the AIRMODE feature is available in the CLI, the current
code only takes BOXAIRMODE into account (triggered by an RC channel).

This PR introduces an isAirmodeActive() function which also returns
true when if the AIRMODE feature has been set, not only when BOXAIRMODE
is active. Checks for IS_RC_MODE_ACTIVE(BOXAIRMODE) have been replaced
by calls to isAirmodeActive().

This will allow adding a simple switch in the configurator, as
reported in https://github.com/iNavFlight/inav-configurator/issues/154.

Eventually, isAirmodeActive() might be modified to always
return true in FW, since there's no reason to disable it in FW
models AFAIK.

src/main/fc/rc_modes.c

@@ -51,6 +51,11 @@ bool isUsingSticksForArming(void)
     return isUsingSticksToArm;
 }
 
+bool isAirmodeActive(void)
+{
+    return feature(FEATURE_AIRMODE) || IS_RC_MODE_ACTIVE(BOXAIRMODE);
+}
+
 #if defined(NAV)
 bool isUsingNavigationModes(void)
 {