Add caching of flash reads

[will-v-pi]

Using picotool info -a takes a while due to multiple flash reads, for the multiple scans for metadata blocks. This could be improved by caching the data read from flash in the picoboot_memory_access class, so you don't need multiple reads from the device of the same data.

[lurch]

Would it be worth caching OTP reads too, or are they always "single shot" as far as picotool is concerned?

[will-v-pi]

Those should always be "single shot" - the OTP commands don't do any "scanning" reads as such, they just read the rows specified, so shouldn't read any rows more than once