base: move /var disk partitioning from overlay.bu to local.bu

The /dev/vdb -> /var partitioning in cookbooks/base/overlay.bu was merged
into every project's main spec, so it ended up in the production/test
ignition even though the second disk (var.qcow2) only exists in the libvirt
dev/test VM. A real host should not have `base` blindly repartition /dev/vdb.

Move it to local.bu / local.bu.template, which feeds local.ign and is merged
into both fcos-dev.ign and fcos-test.ign but is never part of the shipped
package. This restores partitioning for all VMs while keeping it out of
production.

Also add the overlay-examples.bu merge mechanism to scripts/common.mk (mirror
of the existing overlay.bu merge) for future per-project example overlays, and
whitelist overlay-examples.bu in .gitignore.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -2,6 +2,7 @@
 *.ign
 !fcos.bu
 !overlay.bu
+!overlay-examples.bu
 __pycache__/
 .pytest_cache/
 **/build/*.tar.gz

cookbooks/base/overlay.bu

@@ -1,21 +1,6 @@
 variant: fcos
 version: 1.4.0
 storage:
-  disks:
-  - device: /dev/vdb
-    wipe_table: false
-    partitions:
-    # Specify the partition number so that FCOS do not try to create a new
-    # partition at each VM redeployment.
-    - number: 1
-      size_mib: 0
-      start_mib: 0
-      label: var
-  filesystems:
-  - path: /var
-    device: /dev/disk/by-partlabel/var
-    format: xfs
-    with_mount_unit: true
   directories:
   - path: /var/lib/private/sshd
     mode: 0700

local.bu.template

@@ -1,5 +1,21 @@
 variant: fcos
 version: 1.4.0
+storage:
+  disks:
+  - device: /dev/vdb
+    wipe_table: false
+    partitions:
+    # Specify the partition number so that FCOS do not try to create a new
+    # partition at each VM redeployment.
+    - number: 1
+      size_mib: 0
+      start_mib: 0
+      label: var
+  filesystems:
+  - path: /var
+    device: /dev/disk/by-partlabel/var
+    format: xfs
+    with_mount_unit: true
 passwd:
   users:
   - name: john

scripts/common.mk

@@ -451,8 +451,9 @@ build/$(PROJECT_NAME).tar.gz build/$(PROJECT_NAME).bu build/$(PROJECT_NAME)-exam
 		$(SCRIPTS_DIR)/generate-tarball.sh build/$(PROJECT_NAME).tar.gz; \
 		(cat $(SCRIPTS_DIR)/butane.blocklist; echo; for file in $$(find "$$TARGET_CHROOT"); do echo "$${file#$$TARGET_CHROOT}"; done) | sort -u | grep -v -E '^$$' > "$(BUTANE_BLOCKLIST)"; \
 		run make install-examples; \
+		EXAMPLES_YQ_FILES="$$(if [ -f "overlay-examples.bu" ]; then echo "- overlay-examples.bu"; else echo "-"; fi)"; \
 		echo "generate-butane-spec.sh build/$(PROJECT_NAME)-examples.bu"; \
-		$(SCRIPTS_DIR)/generate-butane-spec.sh build/$(PROJECT_NAME)-examples.bu; \
+		$(SCRIPTS_DIR)/generate-butane-spec.sh | yq eval-all '. as $$item ireduce ({}; . *+ $$item)' $$EXAMPLES_YQ_FILES > build/$(PROJECT_NAME)-examples.bu; \
 		(cat $(SCRIPTS_DIR)/butane.blocklist; echo; for file in $$(find "$$TARGET_CHROOT"); do echo "$${file#$$TARGET_CHROOT}"; done) | sort -u | grep -v -E '^$$' > "$(BUTANE_BLOCKLIST)"; \
 	fi
 .PHONY: build/$(PROJECT_NAME).bu build/$(PROJECT_NAME)-examples.bu