[efi] Build iPXE script support into EFI binaries by default

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/defaults/efi.h

 #define SMBIOS_EFI
 
 #define	IMAGE_EFI		/* EFI image support */
+#define	IMAGE_SCRIPT		/* iPXE script image support */
 
 #endif /* CONFIG_DEFAULTS_EFI_H */