make: note COSI vs baremetal-image ambiguity on .cosi targets

The pattern rule passes the file extension as --output-type, so a .cosi
target always resolves to OutputFormat.COSI even for configs that intend
OutputFormat.BAREMETAL_IMAGE (both use extension cosi). Echo a note on
.cosi builds pointing users who want a baremetal-image COSI to invoke
testimages.py directly with --output-type baremetal-image.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bfjelds

Makefile

@@ -950,6 +950,7 @@ validate-pipeline-website-artifact:
 artifacts/%.cosi artifacts/%.iso artifacts/%.vhdx artifacts/%.vhd artifacts/%.qcow2: $$(shell ./tests/images/testimages.py dependencies $$*)
 	@echo "Building '$*' [$@] from $<"
 	@echo "Extension is: $(subst .,,$(suffix $@))"
+	@test "$(subst .,,$(suffix $@))" != "cosi" || echo "NOTE: customizing a COSI image (OutputFormat.COSI). If you intend a baremetal-image COSI (OutputFormat.BAREMETAL_IMAGE), invoke ./tests/images/testimages.py build $* --output-type baremetal-image directly."
 	@echo "Prerequisites:"
 	@echo "$^" | tr ' ' '\n' | sed 's/^/    /'
 	@echo "Building image..."