intel_adsp: strip rimage main.mod when CONFIG_BUILD_OUTPUT_STRIPPED

Be consistency with zephyr.strip

This will help with reproducibility issues like the one in
https://github.com/thesofproject/sof-bin/pull/106



Use the `strip_command` introduced by commit c060b075 ("cmake:
toolchain: bintools abstraction")

boot.mod is already deterministic because it has no debug symbols; no
need to strip it.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>