Bootloader: calculate image hashes while waiting for touch to come up

[matejcik]

Hashing the firmware image takes considerable time, 450 ms per pass on the STM32F4. Touch controller has a 400 ms startup time.

We can initialize the touch controller, start hashing, and when we come back to touch, it's already booted up. (if the hashing is faster on the U5, we'll just be done sooner and wait only up to the 400ms total elapsed time mark)