Allow setting --boot_variable_file in build-ota-package-target

For ota generation via build-ota-package-target, match how
--oem_settings works, but for --boot_variable_file.

Bug: 335732867
Test: Set BOOT_VAR_OTA_CONFIG, make dist, and check ota_metadata
Change-Id: I526e2b280eb288f3abaace2a5daac97a0f7ac6d7
diff --git a/core/Makefile b/core/Makefile
index e6a5bc0..e563873 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -6919,6 +6919,7 @@
         --verbose \
         --path $(HOST_OUT) \
         $(if $(OEM_OTA_CONFIG), --oem_settings $(OEM_OTA_CONFIG)) \
+        $(if $(BOOT_VAR_OTA_CONFIG), --boot_variable_file $(BOOT_VAR_OTA_CONFIG)) \
         $(2) \
         $(patsubst %.zip,%,$(BUILT_TARGET_FILES_PACKAGE)) $(1)
 endef