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>

arch/xtensa/boot/boot-elf/Makefile

@@ -15,6 +15,7 @@ export CPPFLAGS_boot.lds += -P -C
 export KBUILD_AFLAGS += -mtext-section-literals
 
 boot-y		:= bootstrap.o
+targets		+= $(boot-y) boot.lds
 
 OBJS		:= $(addprefix $(obj)/,$(boot-y))
 

arch/xtensa/boot/boot-redboot/Makefile

@@ -13,6 +13,7 @@ endif
 LD_ARGS	= -T $(srctree)/$(obj)/boot.ld
 
 boot-y	:= bootstrap.o
+targets	+= $(boot-y)
 
 OBJS	:= $(addprefix $(obj)/,$(boot-y))
 LIBS	:= arch/xtensa/boot/lib/lib.a arch/xtensa/lib/lib.a