ARC: build: add boot_targets to PHONY

ARC: build: add boot_targets to PHONY

The top-level boot_targets (uImage and uImage.*) should be phony
targets. They just let Kbuild descend into arch/arc/boot/ and create
files there.

If a file exists in the top directory with the same name, the boot
image will not be created.

You can confirm it by the following steps:

  $ export CROSS_COMPILE=<your-arc-compiler-prefix>
  $ make -s ARCH=arc defconfig all   # vmlinux will be built
  $ touch uImage.gz
  $ make ARCH=arc uImage.gz
  CALL    scripts/atomic/check-atomics.sh
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h
  # arch/arc/boot/uImage.gz is not created

Specify the targets as PHONY to fix this.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>