Show 'update complete' message after bootloader update so as not to freak people out

apps/fwupdate/custom.html

@@ -388,6 +388,7 @@ function createJS_bootloader(binary, startAddress, endAddress) {
   for (var i=startAddress;i<endAddress;i+=4096)
     hexJS += 'f.erasePage(0x'+i.toString(16)+');\n';
   hexJS += `f.write(_fw,${startAddress});\n`;
+  hexJS += 'E.showMessage("Update Complete.")\n';
   hexJS += `})()\n`;
   log("DFU ready for upload");
 }