Add configurable boot grace time

This guards against the case when the RX takes longer to obtain a valid
link than the gyro takes to calibrate (previously this delay is all that
stopped immediate arming).

src/main/fc/config.h

@@ -72,6 +72,7 @@ typedef struct systemConfig_s {
 #if defined(STM32F4) && !defined(DISABLE_OVERCLOCK)
     uint8_t cpu_overclock;
 #endif
+    uint8_t powerOnArmingGraceTime; // in seconds
     char boardIdentifier[sizeof(TARGET_BOARD_IDENTIFIER) + 1];
 } systemConfig_t;
 #endif