build: install-dtb: fix race condition when copying dtb

build: install-dtb: fix race condition when copying dtb

Currently for at91 target, Build/install-dtb can be triggered concurrently for
multiple different TARGET_FILESYSTEMS, cp command can fail when the
target file is already open exclusively by other cp process

	[ -f /builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/at91sam9263ek-uImage -a -f /builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/root.ubifs+fs=-m_2048_-e_126KiB_-c_2048+pkg=68b329da ]
	[ -f /builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/at91sam9263ek-uImage -a -f /builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/root.ext4+pkg=68b329da ]
	cp -fpR /builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/linux-4.14.141/arch/arm/boot/dts/at91sam9263ek.dtb /builder/shared-workdir/build/bin/targets/at91/sam9x/openwrt-at91-sam9x-at91sam9263ek.dtb;
	cp -fpR /builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/linux-4.14.141/arch/arm/boot/dts/at91sam9263ek.dtb /builder/shared-workdir/build/bin/targets/at91/sam9x/openwrt-at91-sam9x-at91sam9263ek.dtb;
	cp: cannot create regular file '/builder/shared-workdir/build/bin/targets/at91/sam9x/openwrt-at91-sam9x-at91sam9263ek.dtb': File exists
	Makefile:87: recipe for target '/builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/tmp/openwrt-at91-sam9x-at91sam9263ek-ubifs-dtb' failed
	make[4]: *** [/builder/shared-workdir/build/build_dir/target-arm_arm926ej-s_musl_eabi/linux-at91_sam9x/tmp/openwrt-at91-sam9x-at91sam9263ek-ubifs-dtb] Error 1

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>