[menu] Add menu commands

Allow iPXE scripts to create menus.  For example:

    #!ipxe

    menu iSCSI boot demonstration
    item install         Install Fedora to ${root-path}
    item --default boot  Boot from ${root-path}
    item shell           Enter iPXE shell
    item exit            Exit to BIOS
    choose label && goto ${label}

    :boot
    sanboot ${root-path}

    :install
    sanhook ${root-path}
    chain http://${next-server}/fedora.ipxe

    :shell
    shell

    :exit

Inspired-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/parseopt.c

@@ -26,6 +26,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <errno.h>
 #include <getopt.h>
 #include <ipxe/netdevice.h>
+#include <ipxe/menu.h>
 #include <ipxe/parseopt.h>
 
 /** @file
@@ -113,6 +114,29 @@ int parse_netdev ( const char *text, struct net_device **netdev ) {
 	return 0;
 }
 
+/**
+ * Parse menu name
+ *
+ * @v text		Text
+ * @ret menu		Menu
+ * @ret rc		Return status code
+ */
+int parse_menu ( const char *text, struct menu **menu ) {
+
+	/* Find menu */
+	*menu = find_menu ( text );
+	if ( ! *menu ) {
+		if ( text ) {
+			printf ( "\"%s\": no such menu\n", text );
+		} else {
+			printf ( "No default menu\n" );
+		}
+		return -ENOENT;
+	}
+
+	return 0;
+}
+
 /**
  * Parse flag
  *