tile: fix dependency and .*.cmd inclusion for incremental build

Build targets using if_changed(_rule) must depend on FORCE so that
they are evaluated every time.

In order to include .*.cmd files correctly, build targets added to
"targets" must not be prefixed with $(obj)/ because it is done by
scripts/Makefile.lib .

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