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>

include/image.mk

@@ -44,6 +44,7 @@ IMG_PREFIX_VERCODE:=$(if $(CONFIG_VERSION_CODE_FILENAMES),$(call sanitize,$(VERS
 IMG_PREFIX:=$(VERSION_DIST_SANITIZED)-$(IMG_PREFIX_VERNUM)$(IMG_PREFIX_VERCODE)$(IMG_PREFIX_EXTRA)$(BOARD)$(if $(SUBTARGET),-$(SUBTARGET))
 IMG_ROOTFS:=$(IMG_PREFIX)-rootfs
 IMG_COMBINED:=$(IMG_PREFIX)-combined
+IMG_PART_SIGNATURE:=$(shell echo $(SOURCE_DATE_EPOCH)$(LINUX_VERMAGIC) | mkhash md5 | head --bytes 8)
 
 MKFS_DEVTABLE_OPT := -D $(INCLUDE_DIR)/device_table.txt