build: image: add common and reproducible IMG_PART_SIGNATURE variable

build: image: add common and reproducible IMG_PART_SIGNATURE variable

x86, mvebu and tegra 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 variable which could be
reused by all targets.  While at it, make the content of the variable
reproducible.

Ref: http://lists.infradead.org/pipermail/openwrt-devel/2019-March/016148.html
Signed-off-by: Paul Spooren <mail@aparcar.org>
Suggested-by: Jo-Philipp Wich <jo@mein.io>
[renamed to IMG_PARTSIGNATURE, reworked with epoch+vermagic hash]
Signed-off-by: Petr Štetiar <ynezz@true.cz>