[dhcp] Split PXE menuing code out of dhcp.c

The DHCP client code now implements only the mechanism of the DHCP and
PXE Boot Server protocols.  Boot Server Discovery can be initiated
manually using the "pxebs" command.  The menuing code is separated out
into a user-level function on a par with boot_root_path(), and is
entered in preference to a normal filename boot if the DHCP vendor
class is "PXEClient" and the PXE boot menu option exists.

src/include/usr/autoboot.h

@@ -7,9 +7,15 @@
  *
  */
 
+#include <gpxe/in.h>
+struct net_device;
+
 extern int shutdown_exit_flags;
 
 extern void autoboot ( void );
+extern int boot_next_server_and_filename ( struct in_addr next_server,
+					   const char *filename );
 extern int boot_root_path ( const char *root_path );
+extern int pxe_menu_boot ( struct net_device *netdev );
 
 #endif /* _USR_AUTOBOOT_H */