xtensa: add uImage and xipImage to targets

uImage and xipImage are always rebuilt in the xtensa kernel build
process. Add them to 'targets' to avoid that.
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/boot/Makefile

@@ -18,6 +18,7 @@ export BIG_ENDIAN
 
 subdir-y	:= lib
 targets		+= vmlinux.bin vmlinux.bin.gz
+targets		+= uImage xipImage
 
 # Subdirs for the boot loader(s)