gemini: image: fix race condition when building copy-kernel.bin

gemini: image: fix race condition when building copy-kernel.bin

Make treat copy-kernel.o as intermediate and delete it when it's no
longer needed.  This can fail when the same submake was triggered
multiple times for different devices.

	arm-openwrt-linux-muslgnueabi-as  -k -o copy-kernel.o copy-kernel.S
	export MAKEFLAGS= ;make -w -C copy-kernel CROSS_COMPILE=arm-openwrt-linux-muslgnueabi-
	arm-openwrt-linux-muslgnueabi-objcopy -O binary -S copy-kernel.o copy-kernel.bin
	make[5]: Entering directory '/builder/shared-workdir/build/target/linux/gemini/image/copy-kernel'
	arm-openwrt-linux-muslgnueabi-objcopy -O binary -S copy-kernel.o copy-kernel.bin
	rm copy-kernel.o
	make[5]: Leaving directory '/builder/shared-workdir/build/target/linux/gemini/image/copy-kernel'
	# "App" partition is the rootfs
	arm-openwrt-linux-muslgnueabi-objcopy: 'copy-kernel.o': No such file
	Makefile:27: recipe for target 'copy-kernel.bin' failed
	make[5]: Leaving directory '/builder/shared-workdir/build/target/linux/gemini/image/copy-kernel'
	make[5]: *** [copy-kernel.bin] Error 1
	Makefile:244: recipe for target '/builder/shared-workdir/build/build_dir/target-arm_fa526_musl_eabi/linux-gemini/tmp/openwrt-gemini-storlink_sl93512r-ext4-factory.bin' failed
	make[4]: *** [/builder/shared-workdir/build/build_dir/target-arm_fa526_musl_eabi/linux-gemini/tmp/openwrt-gemini-storlink_sl93512r-ext4-factory.bin] Error 2

With this change, output files are directed to $(KDIR)

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>