nrf/modules/scripts: Add file system formatting script.

Add a helper script _mkfs.py which automatically formats the file system if
nrf.Flash() is located and a VFS file system has been included in the
compilation.

The precedence is: first LFS1, LFS2 then FAT.

ports/nrf/modules/scripts/_mkfs.py

+import uos, nrf
+
+try:
+    from uos import VfsLfs1
+
+    uos.VfsLfs1.mkfs(nrf.Flash())
+except ImportError:
+    try:
+        from uos import VfsLfs2
+
+        uos.VfsLfs2.mkfs(nrf.Flash())
+    except ImportError:
+        try:
+            from uos import VfsFat
+
+            uos.VfsFat.mkfs(nrf.Flash())
+        except ImportError:
+            pass
+        except OSError as e:
+            if e.args[0] == 5:  # I/O Error
+                flashbdev_size = (nrf.Flash.ioctl(4, 0) * nrf.Flash.ioctl(5, 0)) // 1024
+                print()
+                print("Is `FS_SIZE=%iK` enough for FAT filesystem?" % flashbdev_size)