build: Introduce building of artifacts

build: Introduce building of artifacts

We currently could (ab)use IMAGES for this task, but the downside is,
that the filenames has filesystem tied to the filename, which might be
confusing as the artifact itself don't has to be used with that specific
filesystem. Another downside is, that the artifacts built with IMAGES
target are build for every FILESYSTEMS filesystem.

Consider following use case:

 define Device/apalis
   ...
   FILESYSTEMS := ext4 squashfs
   IMAGES := spl-uboot.bin recovery.scr
   IMAGE/spl-uboot.bin := append-uboot-spl | pad-to 68k | append-uboot
   IMAGE/recovery.scr := recovery-scr
 endef

Where we would get target binaries with following filenames:

 openwrt-imx6-apalis-squashfs.recovery.scr
 openwrt-imx6-apalis-squashfs.spl-uboot.bin
 openwrt-imx6-apalis-ext4.recovery.scr
 openwrt-imx6-apalis-ext4.spl-uboot.bin

With proposed patch, we could now just do:

 define Device/apalis
   ...
   ARTIFACTS := spl-uboot.bin recovery.scr
   ARTIFACT/spl-uboot.bin := append-uboot-spl | pad-to 68k | append-uboot
   ARTIFACT/recovery.scr := recovery-scr
 endef

Which would produce target binaries with following filenames:

 openwrt-imx6-apalis-recovery.scr
 openwrt-imx6-apalis-spl-uboot.bin

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
(backported from 493c9a35516c27a8ec412d97e63c8cf6f41a57ea)