[efi] Add last_opened_snpdev()

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

src/include/ipxe/efi/efi_snp.h

 
 extern int efi_snp_hii_install ( struct efi_snp_device *snpdev );
 extern void efi_snp_hii_uninstall ( struct efi_snp_device *snpdev );
+extern struct efi_snp_device * last_opened_snpdev ( void );
 
 #endif /* _IPXE_EFI_SNP_H */

src/interface/efi/efi_snp.c

 	.notify = efi_snp_notify,
 	.remove = efi_snp_remove,
 };
+
+/**
+ * Get most recently opened SNP device
+ *
+ * @ret snpdev		Most recently opened SNP device, or NULL
+ */
+struct efi_snp_device * last_opened_snpdev ( void ) {
+	struct net_device *netdev;
+
+	netdev = last_opened_netdev();
+	if ( ! netdev )
+		return NULL;
+
+	return efi_snp_demux ( netdev );
+}