lib: digital signature config option name change

It was reported that DIGSIG is confusing name for digital signature
module. It was suggested to rename DIGSIG to SIGNATURE.

Requested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <jmorris@namei.org>