net/tftp: fix build if CMD_BOOTEFI is not set

Fixes:
net/tftp.c:811: undefined reference to `efi_set_bootdev'

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/net/tftp.c b/net/tftp.c
index a5ed8c5..6671b1f 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -805,7 +805,9 @@
 		printf("Load address: 0x%lx\n", load_addr);
 		puts("Loading: *\b");
 		tftp_state = STATE_SEND_RRQ;
+#ifdef CONFIG_CMD_BOOTEFI
 		efi_set_bootdev("Net", "", tftp_filename);
+#endif
 	}
 
 	time_start = get_timer(0);