image.mk: remove device_ from manifest filename

image.mk: remove device_ from manifest filename

The manifest file is based on IMG_PREFIX and PROFILE_SANITIZED, whereas
the latter takes a string like DEVICE_8dev_carambola2 and sanitizes it.
This behaviour results in a useless "device_" profile-prefix in the
device manifest filename. Now uses *subst* to remove that.

Therefore this patch results more consistent device file names:

openwrt-ath79-generic-8dev_carambola2-initramfs-kernel.bin
openwrt-ath79-generic-8dev-carambola2.manifest
openwrt-ath79-generic-8dev_carambola2-squashfs-sysupgrade.bin

instead of a single file being called

openwrt-ath79-generic-device_8dev-carambola2.manifest

Signed-off-by: Paul Spooren <mail@aparcar.org>