x86: image: remove TARGET_IMAGES_PAD option and force padding

x86: image: remove TARGET_IMAGES_PAD option and force padding

Currently we're producing unusable x86 combined squashfs images
(18.06.1, 18.06.2 and snapshots) as we don't enable TARGET_IMAGES_PAD,
thus providing very small space for the overlay filesystem, leading to
the following with OpenWrt 18.06.1 r7258-5eb055306f images on x86 QEMU:

 root@(none):/# mount | egrep 'root|overlay'
  /dev/root on /rom type squashfs
  /dev/loop0 on /overlay type ext4
  overlayfs:/overlay on / type overlay

 root@(none):/# df -h | egrep 'root|overlay|Size'
  Filesystem                Size      Used Available Use% Mounted on
  /dev/root                 2.5M      2.5M         0 100% /rom
  /dev/loop0              113.0K      8.0K     97.0K   8% /overlay
  overlayfs:/overlay      113.0K      8.0K     97.0K   8% /

We should rather ensure proper image padding in image generation code
and we shouldn't rely on config options in order to generate usable
images.

So this patch removes TARGET_IMAGES_PAD config option and enforces
padding on combined images.

Signed-off-by: Petr Štetiar <ynezz@true.cz>