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>

target/linux/x86/image/Makefile

@@ -80,7 +80,7 @@ ifneq ($(CONFIG_GRUB_IMAGES),)
 		-e 's#@TITLE@#$(GRUB_TITLE)#g' \
 		./grub.cfg > $(KDIR)/root.grub/boot/grub/grub.cfg
 	-$(CP) $(STAGING_DIR_ROOT)/boot/. $(KDIR)/root.grub/boot/
-	PADDING="$(CONFIG_TARGET_IMAGES_PAD)" SIGNATURE="$(SIGNATURE)" PATH="$(TARGET_PATH)" $(SCRIPT_DIR)/gen_image_generic.sh \
+	PADDING="1" SIGNATURE="$(SIGNATURE)" PATH="$(TARGET_PATH)" $(SCRIPT_DIR)/gen_image_generic.sh \
 		$(BIN_DIR)/$(IMG_PREFIX)-combined-$(1).img \
 		$(CONFIG_TARGET_KERNEL_PARTSIZE) $(KDIR)/root.grub \
 		$(CONFIG_TARGET_ROOTFS_PARTSIZE) $(KDIR)/root.$(1) \