mkimage: Build signing only if board has CONFIG_FIT_SIGNATURE

At present mkimage is set up to always build with image signing support.
This means that the SSL libraries (e.g. libssl-dev) are always required.

Adjust things so that mkimage can be built with and without image signing,
controlled by the presence of CONFIG_FIT_SIGNATURE in the board config file.

If CONFIG_FIT_SIGNATURE is not enabled, then mkimage will report a warning
that signing is not supported. If the option is enabled, but libraries are
not available, then a build error similar to this will be shown:

lib/rsa/rsa-sign.c:26:25: fatal error: openssl/rsa.h: No such file or directory

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/Makefile b/tools/Makefile
index cc912fb..46159b2 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -126,7 +126,7 @@
 LIBFDT_OBJ_FILES-y += fdt_wip.o
 
 # RSA objects
-RSA_OBJ_FILES-y += rsa-sign.o
+RSA_OBJ_FILES-$(CONFIG_FIT_SIGNATURE) += rsa-sign.o
 
 # Generated LCD/video logo
 LOGO_H = $(OBJTREE)/include/bmp_logo.h