sh: fix build error for empty CONFIG_BUILTIN_DTB_SOURCE

If CONFIG_USE_BUILTIN_DTB is enabled, but CONFIG_BUILTIN_DTB_SOURCE
is empty (for example, allmodconfig), it fails to build, like this:

  make[2]: *** No rule to make target 'arch/sh/boot/dts/.dtb.o',
  needed by 'arch/sh/boot/dts/built-in.o'.  Stop.

Surround obj-y with ifneq ... endif.

I replaced $(CONFIG_USE_BUILTIN_DTB) with 'y' since this is always
the case from the following code from arch/sh/Makefile:

  core-$(CONFIG_USE_BUILTIN_DTB)  += arch/sh/boot/dts/

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>