[configs/alter] Miscellaneous fixup

* root-image: use ext4 instead of ext2
* Restore the extra EFI support, except the efiboot.img for "El Torito" EFI boot mode(as it takes up 31MiB)
* New packages in packages.{backport,both}: pax, bmon, 6rdctl-git and so on
* Use the emacs-friendly US keymap
* Remove the empty fstab in root-image/etc
  ... and other minor fixup