mkimge: Reject signing-related flags without FIT_SIGNATURE

When CONFIG_FIT_SIGNATURE=n the signing options are not available. If a
user is careful they will notice this when looking at the help output.

If they are not careful they will waste several hours wondering why
their FIT doesn't contain a /signature node, as mkimage will silently
ingore the signing related options.

Make it obvious that the commands don't work by removing them from the
getopt opt_string.

 $ mkimage -f machine.its -k keys -K u-boot-pubkey.dtb -r image.fit
 mkimage: invalid option -- 'k'
 Error: Invalid option

Signed-off-by: Joel Stanley <joel@jms.id.au>