FW: Optionally start app from flash

[mchack-work]

Change firmware to optionally load a device app from flash. Note: Should probably be used only on some products, not necessarily all TKey products.

Firmware first checks on some signal from FPGA if it's supposed to wait for a device app to be loaded the normal way.

Firmware needs to be able to read a small header on flash and if
found load application from flash memory, doing the normal measuring of the device app.

We will probably need some kind of header on the flash containing some magic cookie to tell the firmware that a pre-loaded app is available.

We might want to add the app digest to the header, so the firmware can compare the computed digest to the expected digest, which is basically what we do in client apps right now, too.

Depends on

• FPGA support for flash access Hardware support to access flash from CPU #213