kbuild: do not print extra logs for V=2

Some scripts increase the verbose level when V=1, but others when
not V=0.

I think the former is correct because V=2 is not a log level but
a switch to print the reason for rebuilding.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

scripts/kernel-doc

@@ -175,8 +175,8 @@ my $declaration_start_line;
 my ($type, $declaration_name, $return_type);
 my ($newsection, $newcontents, $prototype, $brcount, %source_map);
 
-if (defined($ENV{'KBUILD_VERBOSE'})) {
-	$verbose = "$ENV{'KBUILD_VERBOSE'}";
+if (defined($ENV{'KBUILD_VERBOSE'}) && $ENV{'KBUILD_VERBOSE'} =~ '1') {
+	$verbose = 1;
 }
 
 if (defined($ENV{'KCFLAGS'})) {