make/core/Makefile: Skip images for local builds by default.

Packing and archiving the image are expensive operations.
Swap the image archive to dist-files for builders, and skip
building the raw image locally unless it's specifically
requested via the new PHONY targets.

Bug: None
Test: m, m dist, m pack-image
Change-Id: I2a3f1ebea689c23da88ca6d0757b827b7eb0c936