[efi] Mark EFI debug transcription functions as __attribute__ (( pure ))

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

src/include/ipxe/efi/efi.h

 extern EFI_DEVICE_PATH_PROTOCOL *efi_loaded_image_path;
 extern EFI_SYSTEM_TABLE *efi_systab;
 
-extern const char * efi_guid_ntoa ( EFI_GUID *guid );
-extern const char * efi_devpath_text ( EFI_DEVICE_PATH_PROTOCOL *path );
-extern const char * efi_handle_name ( EFI_HANDLE handle );
+extern const __attribute__ (( pure )) char * efi_guid_ntoa ( EFI_GUID *guid );
+extern const __attribute__ (( pure )) char *
+efi_devpath_text ( EFI_DEVICE_PATH_PROTOCOL *path );
+extern const __attribute__ (( pure )) char *
+efi_handle_name ( EFI_HANDLE handle );
 
 extern void dbg_efi_openers ( EFI_HANDLE handle, EFI_GUID *protocol );
 extern void dbg_efi_protocols ( EFI_HANDLE handle );

src/interface/efi/efi_debug.c

  * @v guid		GUID
  * @ret string		Printable string
  */
-const char * efi_guid_ntoa ( EFI_GUID *guid ) {
+const __attribute__ (( pure )) char * efi_guid_ntoa ( EFI_GUID *guid ) {
 	union {
 		union uuid uuid;
 		EFI_GUID guid;
  * @v path		Device path
  * @ret text		Textual representation of device path, or NULL
  */
-const char * efi_devpath_text ( EFI_DEVICE_PATH_PROTOCOL *path ) {
+const __attribute__ (( pure )) char *
+efi_devpath_text ( EFI_DEVICE_PATH_PROTOCOL *path ) {
 	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
 	static char text[256];
 	void *start;
  * @v handle		EFI handle
  * @ret text		Name of handle, or NULL
  */
-const char * efi_handle_name ( EFI_HANDLE handle ) {
+const __attribute__ (( pure )) char * efi_handle_name ( EFI_HANDLE handle ) {
 	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
 	struct efi_handle_name_type *type;
 	unsigned int i;