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>