[configs/alter] Miscellaneous fixup

* Set the default serial speed to 115200 baud, also add a new boot entry for this
* Restore the extra EFI support, except for the efiboot.img, which is used for "El Torito" EFI boot mode
* Use the emacs-friendly US keymap
* root-image: use ext4 instead of ext2
* New packages in packages.{backport,both}: pax, bmon, 6rdctl-git, autodkms, smp_utils, isz-tool and so on
* build.sh: add '--preserve=timestamps' to some cp command, in order to preserve the mtime
* Change the way to determine if enable NoCow(chattr +C) for btrfs pool
* Remove the empty fstab in root-image/etc
  ... and other minor fixup