sign-file: Document dependency on OpenSSL devel libraries

The revised sign-file program is no longer a script that wraps the openssl
program, but now rather a program that makes use of the OpenSSL's crypto
library.  This means that to build the sign-file program, the kernel build
process now has a dependency on openssl-devel in addition to openssl.

Document this in Kconfig and in module-signing.txt.
Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/module-signing.txt

@@ -111,6 +111,9 @@ This has a number of options available:
      additional certificates which will be included in the system keyring by
      default.
 
+Note that enabling module signing adds a dependency on the OpenSSL devel
+packages to the kernel build processes for the tool that does the signing.
+
 
 =======================
 GENERATING SIGNING KEYS

init/Kconfig

@@ -1897,6 +1897,9 @@ config MODULE_SIG
 	  is simply appended to the module. For more information see
 	  Documentation/module-signing.txt.
 
+	  Note that this option adds openssl-devel as a kernel build dependency
+	  so that the signing tool can use its crypto library.
+
 	  !!!WARNING!!!  If you enable this option, you MUST make sure that the
 	  module DOES NOT get stripped after being signed.  This includes the
 	  debuginfo strip done by some packagers (such as rpmbuild) and