spiffs_pgobj_ndxheader_s: Add a missing alignment

Withtout this change, I was not able to read images built
with the following tools:

    * https://github.com/igrr/mkspiffs
    * spiffsgen.py from ESP-IDF

Introduce a new Kconfig option, SPIFFS_COMPAT_OLD_NUTTX,
for those who prefer to keep the on-disk (bug-to-bug?) compatibility
with the older NuttX.

fs/spiffs/Kconfig

@@ -176,4 +176,13 @@ config SPIFFS_COPYBUF_STACK
 		value generates more read/writes.  No meaning having it bigger
 		than logical page size.
 
+config SPIFFS_COMPAT_OLD_NUTTX
+	bool "Compatibility with older NuttX SPIFFS"
+	default n
+	---help---
+		Enable the compatiblilty with older NuttX versions.
+		(Older than NuttX 11.0.0, where this option was introduced.)
+
+		Note: This config affects the on-flash structure.
+
 endif # FS_SPIFFS

fs/spiffs/src/spiffs_core.h

@@ -400,6 +400,10 @@ begin_packed_struct struct spiffs_page_header_s
 begin_packed_struct struct spiffs_pgobj_ndxheader_s
 {
   struct spiffs_page_header_s phdr;     /* common page header */
+#ifndef CONFIG_SPIFFS_COMPAT_OLD_NUTTX
+  uint8_t _align[4 - ((sizeof(struct spiffs_page_header_s) & 3) ==
+                 0 ? 4 : (sizeof(struct spiffs_page_header_s) & 3))];
+#endif
   uint32_t size;                        /* size of object */
   uint8_t type;                         /* type of object */
   uint8_t name[CONFIG_SPIFFS_NAME_MAX]; /* name of object */