Today I got Lumia 520 "ERROR : Unable to find a bootable option. press any key to shutdown".
This ia common trouble when upgrading WP 8.0 to 8.1, and this is
very easy to Fix, by EasyJTAG
While other box failed to detect "ivalid blablabla", Easy Jtag show the power
Here's step by step how to: 
1.) Solder only 4 wire : CMD, CLK, DAT0, and GND
2.) Open Easyjtag Emmc tool, n connet usb directly to pcb as power of Vcc&vccq
3. Click Emmc Tool, set eMMC Voltage to : 3,3v
Detect eMMC card, you will see in log "Emmc Permanent Write Protection : YES"
This is the problem, just click on Advanced Opt, select "SAMSUNG : remove write protection" and execute by Custom Operation -->
4. Detect Emmc Card again to see the result, the trouble maker is remove Now
, ..
5. Assemble phone, and flash normally with any box u have.
And, just it, you just revive your phone back to live...
Don't forget to Thanks to EasyJTAG