Enable SRXL2 ESC build behind USE_SRXL2_ESC

The SRXL2 ESC driver implementation was guarded by USE_SERIAL, a macro
that is not defined anywhere in INAV. As a result srxl2_esc.c always
compiled the empty stub: the driver never opened a port, never sent
throttle and never read telemetry, so the feature was dead code on every
target.

Introduce a dedicated USE_SRXL2_ESC feature macro (defined for targets
with more than 512 KB flash) and gate the driver and the
pwmCompleteMotorUpdate() call on it instead of the non-existent
USE_SERIAL.

Verified by building SPEEDYBEEF405WING (real implementation now compiled
and linked) and SITL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GroYFi23gHDgj43xrTAVaY

Author: claude

src/main/drivers/srxl2_esc.c

@@ -35,7 +35,7 @@
 
 #include "io/serial.h"
 
-#if defined(USE_SERIAL)
+#if defined(USE_SRXL2_ESC)
 
 #define SRXL2_ESC_BAUDRATE                115200
 #define SRXL2_ESC_FRAME_TIMEOUT_US        500

src/main/fc/fc_core.c

@@ -1042,7 +1042,7 @@ void taskRunRealtimeCallbacks(timeUs_t currentTimeUs)
     afatfs_poll();
 #endif
 
-#if defined(USE_DSHOT) || defined(USE_SERIAL)
+#if defined(USE_DSHOT) || defined(USE_SRXL2_ESC)
     pwmCompleteMotorUpdate();
 #endif
 

src/main/target/common.h

@@ -214,6 +214,7 @@
 #define USE_34CHANNELS
 #define MAX_MIXER_PROFILE_COUNT 2
 #define USE_SMARTPORT_MASTER
+#define USE_SRXL2_ESC
 #ifdef USE_GPS
 #define USE_GEOZONE
 #define MAX_GEOZONES_IN_CONFIG 63