SFU: add USBMSD binding only if included explicitly

Since SFU is being used for OTA on IotCloud, every sketch used to pull in USBMSD and expose a 1MB FAT disk when connected.
This patch fixed this beahviour, providing the hooks only if the user explicitly includes
"PluggableUSBMSD.h"
in the sketch (or in another library)

Author: facchinm

libraries/SFU/src/SFU.cpp

@@ -1,32 +1,26 @@
 #include "SFU.h"
 #include "FlashIAPBlockDevice.h"
-#include "FATFileSystem.h"
+#if __has_include("PluggableUSBMSD.h")
 #include "PluggableUSBMSD.h"
+#define ADD_USB_MSD
+#endif
 
 const unsigned char SFU[0x10000] __attribute__ ((section(".second_stage_ota"), used)) = {
 	#include "rp2040.h"
 };
 
 FlashIAPBlockDevice bd(XIP_BASE + 0xF00000, 0x100000);
 
-void USBMSD::begin()
-{
-    int err = getFileSystem().mount(&bd);
-    if (err) {
-        err = getFileSystem().reformat(&bd);
-    }
-}
-
-mbed::FATFileSystem& USBMSD::getFileSystem()
-{
+mbed::FATFileSystem& SFU::getFileSystem() {
 	static mbed::FATFileSystem fs("ota");
 	return fs;
 }
 
-USBMSD MassStorage(&bd);
-
 int SFU::begin() {
-	MassStorage.begin();
+	int err = getFileSystem().mount(&bd);
+    if (err) {
+        err = getFileSystem().reformat(&bd);
+    }
 }
 
 int SFU::download(const char* url) {
@@ -36,3 +30,19 @@ int SFU::download(const char* url) {
 int SFU::apply() {
 	// No autoreboot
 }
+
+#ifdef ADD_USB_MSD
+
+void USBMSD::begin()
+{
+	SFU::begin();
+}
+
+mbed::FATFileSystem& USBMSD::getFileSystem()
+{
+	return SFU::getFileSystem();
+}
+
+USBMSD MassStorage(&bd);
+
+#endif

libraries/SFU/src/SFU.h

@@ -1,10 +1,12 @@
 #pragma once
 
 #include "WiFiNINA.h"
+#include "FATFileSystem.h"
 
 class SFU {
 public:
 	static int begin();
 	static int download(const char* url);
 	static int apply();
+	static mbed::FATFileSystem& getFileSystem();
 };