images: allow building of non-gateworks images

revert bec4a5e2712540cccbc99f0a47497ed7e7e7f6f2

The multiple ubi configuration capability requires that images inherit
ubi-config and yet still requires a conf file variable to be changed. This
causes issues for non gateworks images.

Instead, the user needs to make sure they have set MKUBIFS_ARGS and
UBINIZE_ARGS properly for their targets.

There are two flash layouts used currently by Gateworks boards:
 - 'normal' for most NAND flash devices
     MKUBIFS_ARGS += "-F -m 2048 -e 124KiB -c 1912 -x zlib"
     UBINIZE_ARGS += "-m 2048 -p 128KiB -s 2048"
 - 'large' for 2GB+ NAND flash devices
     MKUBIFS_ARGS += "-F -m 4096 -e 248KiB -c 8124 -x zlib"
     UBINIZE_ARGS += "-m 4096 -p 256KiB -s 4096"