kbuild: unify cmd_dt_S_dtb and cmd_dt_S_dtbo

cmd_dt_S_dtb and cmd_dt_S_dtbo are almost the same; the only difference
is the prefix of the begin/end symbols. (__dtb vs __dtbo)

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>