Port fat_io_lib, mount atasd0mp1 as sys:

kernel/vfs/vfs.h

@@ -6,6 +6,7 @@
 #include <stdbool.h>
 #include "spinlock/spinlock.h"
 #include "fs/portlfs/portlfs.h"
+#include "fs/portfatfs/portfatfs.h"
 #include "storedev/storedev.h"
 #include "sysdefs/fs.h"
 #include "compiler/attr.h"
@@ -15,10 +16,14 @@
 
 enum {
   VFS_LITTLEFS,
+  VFS_FAT16,
+  VFS_FAT32,
 };
 
 UNUSED static const char *vfs_strings[] = {
   "Little FS",
+  "FAT16",
+  "FAT32",
 };
 
 enum {
@@ -34,7 +39,6 @@ typedef struct VfsObj {
   SpinLock spinlock;
   char path[VFS_MOUNTPOINT_LABEL_MAX+1+VFS_PATH_MAX];
   void *extra;
-  size_t extrasize;
   struct VfsMountPoint *vmp;
   int32_t flags;
   int32_t (*read)(struct VfsObj *vobj, uint8_t *const buffer, size_t n, size_t off);
@@ -65,6 +69,7 @@ typedef struct VfsMountPoint {
 
   union {
     LittleFs littlefs;
+    FatFs fatfs;
   } fs;
   SpinLock spinlock;
 } VfsMountPoint;