fix: detect lowercase makefile in submodule build loop

OMEN uses lowercase `makefile` which was missed by the capital-M check.
On case-sensitive filesystems (Linux) this meant OMEN wouldn't be built
during `make submodules`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Makefile

@@ -12,7 +12,7 @@ submodules:
 	$(MAKE) submodules-pre; \
 	if [ -f .gitmodules ] && command -v git >/dev/null 2>&1; then \
 		for path in $$(git config --file .gitmodules --get-regexp path | awk '{print $$2}'); do \
-			if [ -f "$$path/Makefile" ]; then \
+			if [ -f "$$path/Makefile" ] || [ -f "$$path/makefile" ]; then \
 				$(MAKE) -C "$$path"; \
 			fi; \
 		done; \