certs: move scripts/extract-cert to certs/

extract-cert is only used in certs/Makefile.

Move it there and build extract-cert on demand.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/certs/.gitignore b/certs/.gitignore
index 8c3763f..9e42fe3 100644
--- a/certs/.gitignore
+++ b/certs/.gitignore
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0-only
+/extract-cert
 /x509_certificate_list
 /x509_revocation_list