Not going to happen, I'm afraid. A USB port supplies 500mA at most; that's only 0.5A--and the client device must connect in low-power mode and then tell the host that it wants to switch to high-power mode to get up to the 500mA range. You need almost 6 times more available current capacity than a USB port can supply to provide a 1A 12VDC output, and that's before accounting for losses and the fact that not only will you need to develop an efficient switchmode supply, you'll also need to develop something to tell the host controller to switch to high-power mode. And high-power mode still won't be enough to produce 12V with 1A capacity. Even the new USB 3 spec wouldn't handle it.
It might be possible to use several USB ports in parallel to do the job but that would be an ugly brute-force solution. Just buy a cheap wall adaptor. It will be far easier and cheaper than building your own.