clean up filesystem goo, add spiffs to install scripts, fix #496

@mc-hamster seems to work pretty good for me, so I'll send a PR to you
for the dev-http branch.

I'll push out an android alpha build later today (once the build is
complete).  Once this new device load is out in the field _future_
device builds will support updating spiffs from android. (i.e. device
loads older than 1.1.9 must be updated to 1.1.9 or later before spiffs
support is implemented on the device side - so some users might need to
update twice before the new spiffs contents will appear on their device)

src/FSCommon.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include "configuration.h"
+
+// Cross platform filesystem API
+
+#ifdef PORTDUINO
+// Portduino version
+#include "PortduinoFS.h"
+#define FS PortduinoFS
+#define FSBegin() true
+#define FILE_O_WRITE "w"
+#define FILE_O_READ "r"
+#elif !defined(NO_ESP32)
+// ESP32 version
+#include "SPIFFS.h"
+#define FS SPIFFS
+#define FSBegin() FS.begin(true)
+#define FILE_O_WRITE "w"
+#define FILE_O_READ "r"
+#else
+// NRF52 version
+#include "InternalFileSystem.h"
+#define FS InternalFS
+#define FSBegin() FS.begin()
+using namespace Adafruit_LittleFS_Namespace;
+#endif
+
+void fsInit();