Makefile: Auto-clean board build directories when coreboot canary changes

When the coreboot git commit changes (canary mismatch), remove and recreate
the board-specific build directories to prevent stale artifacts from causing
compilation failures.

This eliminates the need for manual `rm -rf build/x86/BOARD` before rebuilding
after coreboot canary updates in development cycles (when creating patches).

One currently still has to, eg:
echo "bogues repo url + commit hash" | sudo tee /home/user/heads/build/x86/coreboot-25.09/.canary

Current logic then:
1. Detects canary changes and triggers repository cleanup
2. Removes both Heads board directory (build/x86/BOARD) and coreboot board
   directory (build/x86/coreboot-*/BOARD) that contain stale build artifacts
3. Recreates empty board directories for the subsequent build
4. Allows patches to apply cleanly without Ada compilation errors

This ensures Ada builds work correctly after coreboot updates without manual
intervention (resync repo, apply patches, clean artifact dirs and rebuilds only what changed).

Signed-off-by: Thierry Laurion <insurgo@riseup.net>

Author: tlaurion

Makefile

@@ -492,6 +492,10 @@ define define_module =
 		git -C "$(build)/$($1_base_dir)" submodule sync && \
 		echo "INFO: Updating submodules (init and checkout)" && \
 		git -C "$(build)/$($1_base_dir)" submodule update --init --checkout && \
+		echo "INFO: Cleaning board-specific build directories to prevent stale artifacts" && \
+		rm -rf "$(build)/$(BOARD)" "$(build)/$($1_base_dir)/$(BOARD)" && \
+		echo "INFO: Recreating board directories" && \
+		mkdir -p "$(build)/$(BOARD)" "$(build)/$($1_base_dir)/$(BOARD)" && \
 		echo "INFO: Updating .canary file with new repo info" && \
 		echo -n '$($1_repo)|$($1_commit_hash)' > "$$@" ; \
 	fi