MFC: 253676

- Once we have shifted arguments thrice, base-bits-dir is $1 rather than $4.
  Introduce $BASEBITSDIR for clarity and in order to avoid repeating this
  mistake in the future. Fixing this ensures that we pick up the newly built
  boot code and loader native to the target, which is especially relevant
  when cross-building release images.
- It is pointless to specify an endianess for ISO 9660 images so strip that.

Approved by:	re (kib)