build: separate per-platform build artifacts

Move build per-platform artifacts to be entirely under $CANTRIP_OUT_DIR
and make each platform's sim.mk & sim_sel4test.mk more consistent.

Specific changes:
- add EXT_BUILTINS_{DEBUG, RELEASE} for the constructed builtins bundle
  which is moved to $CANTRIP_OUT_DIR
- move the "tmp" directory used to construct bundles under $CANTRIP_OUT_DIR
- change sel4test out directories to use platform instead of target arch
- use make targets for dependencies instead of pathnames to artifacts
  (this simplifies direct building of artifacts)
- fix minisel targets
- use make shell operations to dredge platforms.camkes for cpio parameters
- cleanup/rewrite rpi3 sim.mk

Tested all {shodan,nexus,rpi3} sim targets (several had bitrot'd).

Shodan & nexus sim_sel4test.mk are now identical; leaving a copy for now.

Bug: 293296515

Change-Id: I29b431555f0a142b753afa2d8bde4a46447f7a41