docs: Kbuild/Makefile: allow check for missing docs at build time

While this doesn't make sense for production Kernels, in order to
avoid regressions when documents are touched, let's add a
check target at the make file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/documentation-file-ref-check

@@ -22,9 +22,16 @@ $scriptname =~ s,.*/([^/]+/),$1,;
 # Parse arguments
 my $help = 0;
 my $fix = 0;
+my $warn = 0;
+
+if (! -d ".git") {
+	printf "Warning: can't check if file exists, as this is not a git tree";
+	exit 0;
+}
 
 GetOptions(
 	'fix' => \$fix,
+	'warn' => \$warn,
 	'h|help|usage' => \$help,
 );
 
@@ -139,6 +146,8 @@ while (<IN>) {
 			if (!($ref =~ m/(scripts|Kconfig|Kbuild)/)) {
 				$broken_ref{$ref}++;
 			}
+		} elsif ($warn) {
+			print STDERR "Warning: $f references a file that doesn't exist: $fulref\n";
 		} else {
 			print STDERR "$f: $fulref\n";
 		}