tools/fit_check_sign: make key optional

Allow invoking fit_check_sig without the key parameter, allowing to
validate only checksums and hashes for unsigned images.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
diff --git a/tools/Makefile b/tools/Makefile
index d0e4d2d..97ce1db 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -63,8 +63,7 @@
 hostprogs-y += mkenvimage
 mkenvimage-objs := mkenvimage.o os_support.o generated/lib/crc32.o
 
-hostprogs-y += dumpimage mkimage
-hostprogs-$(CONFIG_TOOLS_LIBCRYPTO) += fit_info fit_check_sign
+hostprogs-y += dumpimage mkimage fit_info fit_check_sign
 hostprogs-$(CONFIG_TOOLS_LIBCRYPTO) += fdt_add_pubkey
 hostprogs-$(CONFIG_TOOLS_LIBCRYPTO) += preload_check_sign