Revert "omni: switch to sha256 checksum"

This reverts commit f0e22647c3aa6f44995ec8fea305d06d0a330c58.

Reason for revert: OpenDelta relies on md5sum

Change-Id: Ic3a56e77dc98d6d6e1e7da0b3e39c9aa12abd566
diff --git a/build/tasks/cookies.mk b/build/tasks/cookies.mk
index d711727..f2ca0a5 100644
--- a/build/tasks/cookies.mk
+++ b/build/tasks/cookies.mk
@@ -1,10 +1,10 @@
 
 OMNI_TARGET_PACKAGE := $(PRODUCT_OUT)/omni-$(ROM_VERSION).zip
 
-SHA256 := prebuilts/build-tools/path/$(HOST_PREBUILT_TAG)/sha256sum
+MD5 := prebuilts/build-tools/path/$(HOST_PREBUILT_TAG)/md5sum
 
 .PHONY: cookies
 cookies: $(INTERNAL_OTA_PACKAGE_TARGET)
 	$(hide) ln -f $(INTERNAL_OTA_PACKAGE_TARGET) $(OMNI_TARGET_PACKAGE)
-	$(hide) $(SHA256) $(OMNI_TARGET_PACKAGE) | sed "s|$(PRODUCT_OUT)/||" > $(OMNI_TARGET_PACKAGE).sha256sum
+	$(hide) $(MD5) $(OMNI_TARGET_PACKAGE) | sed "s|$(PRODUCT_OUT)/||" > $(OMNI_TARGET_PACKAGE).md5sum
 	@echo -e "Package complete: $(OMNI_TARGET_PACKAGE)" >&2