realtek: image: reset COMPILE between devices

realtek: image: reset COMPILE between devices

Device/zyxel_zynos sets COMPILE := loader-$(1).bin to drive the
standalone rt-loader build, but include/image.mk's Device/Init does
not reset COMPILE between TARGET_DEVICES iterations. When a non-zynos
device follows a zynos device, the stale COMPILE entry survives and
Device/Build/compile registers a second recipe for the previous
device's loader-*.bin target. Make warns about the override and the
second (wrong) recipe wins.

Reset COMPILE in Device/Default so each device starts with a clean
slate.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23226
Signed-off-by: Robert Marko <robimarko@gmail.com>