build: use do_not_edit variable where possible

We should not use hardcoded string to warn users about
file was generated by configure script. Since we already
have do_not_edit variable we can use it

PR-URL: nodejs/node#12610
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

configure

@@ -1358,8 +1358,7 @@ if options.prefix:
 
 config = '\n'.join(map('='.join, config.iteritems())) + '\n'
 
-write('config.mk',
-      '# Do not edit. Generated by the configure script.\n' + config)
+write('config.mk', do_not_edit + config)
 
 gyp_args = [sys.executable, 'tools/gyp_node.py', '--no-parallel']