x86,tegra,mvebu: image: use common reproducible IMG_PART_SIGNATURE

x86,tegra,mvebu: image: use common reproducible IMG_PART_SIGNATURE

These targets are currently using more or less same SIGNATURE variable
which provides unique partition ID/signature, so it makes sense to
refactor it out into common IMG_PART_SIGNATURE variable which could be
reused by all targets.

This is another step in the direction of reproducible OpenWrt images.

Signed-off-by: Paul Spooren <mail@aparcar.org>
[split into separate commit, renamed to IMG_PART_SIGNATURE]
Signed-off-by: Petr Štetiar <ynezz@true.cz>