plat: marvell: armada: a3k: Do not modify $(WTMI_MULTI_IMG)

Rather create a temporary copy in $(BUILD_PLAT) and modify only copy.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I256c029106ea6f69faa086fc4e5bee9f68cd257f
1 file changed