Allow a kernel config to specify a set but empty value via

'makeoptions OPTION=' for consistency with the make commandline.

Previously 'makeoptions WERROR=' would result in a syntax error; now
it produces the same effect as 'makeoptions WERROR'.  Both forms now
result in 'WERROR=' in the generated Makefile.

usr.sbin/config/config.y

@@ -261,6 +261,7 @@ Mkopt_list:
 
 Mkoption:
 	Save_id { newopt(&mkopt, $1, ns(""), 0); } |
+	Save_id EQUALS { newopt(&mkopt, $1, ns(""), 0); } |
 	Save_id EQUALS Opt_value { newopt(&mkopt, $1, $3, 0); } |
 	Save_id PLUSEQUALS Opt_value { newopt(&mkopt, $1, $3, 1); } ;