rules.mk: introduce new variable OUTPUT_DIR

Introduce a new variable OUTPUT_DIR which refers to the parent bin/ directory
without the architecture specific component.

Also construct the BIN_DIR variable based on OUTPUT_DIR now and add both target
and subtarget components to it to prevent different subtargets from
overwriting each others files in bin/.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>