xtensa: add boot subdirectories build artifacts to 'targets'

Xtensa always rebuilds the following even if nothing in the source code
has been changed. Passing V=2 shows the reason.

  AS      arch/xtensa/boot/boot-elf/bootstrap.o - due to bootstrap.o not in $(targets)
  LDS     arch/xtensa/boot/boot-elf/boot.lds - due to boot.lds not in $(targets)

They are built by if_changed(_dep). Add them to 'targets' so .*.cmd files
are included.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20200722004707.779601-1-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>