AP_Filesystem: add littlefs based filesystem support

use correct read status for nor flash
implement format on littlefs
optimize device calls in littlefs flash usage
check for fileops allowed in littlefs
littlefs optimization and support for mtime

libraries/AP_Filesystem/AP_Filesystem.cpp

@@ -35,6 +35,9 @@ static AP_Filesystem_ESP32 fs_local;
 #elif AP_FILESYSTEM_POSIX_ENABLED
 #include "AP_Filesystem_posix.h"
 static AP_Filesystem_Posix fs_local;
+#elif AP_FILESYSTEM_LITTLEFS_ENABLED
+#include "AP_Filesystem_FlashMemory_LittleFS.h"
+static AP_Filesystem_FlashMemory_LittleFS fs_local;
 #else
 static AP_Filesystem_Backend fs_local;
 int errno;
@@ -310,6 +313,12 @@ void AP_Filesystem::unmount(void)
     return LOCAL_BACKEND.fs.unmount();
 }
 
+// if non-zero size at which syncs should be peformed, only used by flash fs
+uint32_t AP_Filesystem::get_sync_size(void) const
+{
+    return LOCAL_BACKEND.fs.get_sync_size();
+}
+
 /*
   Load a file's contents into memory. Returned object must be `delete`d to free
   the data. The data is guaranteed to be null-terminated such that it can be

libraries/AP_Filesystem/AP_Filesystem.h

@@ -128,6 +128,9 @@ class AP_Filesystem {
     // format filesystem.  This is async, monitor get_format_status for progress
     bool format(void);
 
+    // if non-zero size at which syncs should be peformed, only used by flash fs
+    uint32_t get_sync_size() const;
+
     // retrieve status of format process:
     AP_Filesystem_Backend::FormatStatus get_format_status() const;