Add an option to verify the flash contents after flashing

[jessebraham]

See:
https://github.com/espressif/esptool/blob/590c2c68381b32e1d296d0b338b9d50a231169a5/esptool/cmds.py#L1135-L1173

[bugadani]

It would also be nice if we had the option to save a partition to a file. If verification requires reading back from flash, it probably shouldn't be very hard to add either function if the other exists :)

[SergioGasquez]

Related esptool docs: https://docs.espressif.com/projects/esptool/en/latest/esp32/advanced-topics/serial-protocol.html#verifying-uploaded-data