build: remove TARGET_IMAGES_PAD option

build: remove TARGET_IMAGES_PAD option

It's being used only in x86 target to produce combined images, where
it's mandatory to have padded images in order to produce working
squashfs combined images usable in QEMU.

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% /

So 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.

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