kbuild: change DTC_FLAGS_<basetarget>.o to take the path relative to $(obj)

For the same rationale as commit 54b8ae66 ("kbuild: change
*FLAGS_<basetarget>.o to take the path relative to $(obj)").

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