Improve flash speed by skipping bootloader and partition tables when possible

[MabezDev]

See: esp-rs/esp-hal#803 (comment)

But essentially, we could make espflash smart enough to flash the things that have changed, which is usually just the main app.

[bugadani]

If it is possible to read the hash of a particular flash region, it should also be possible I believe to implement selective reflashing: for each flash block, read the hash, compare, and only reflash if differs.

Also, cc #259 as this issue is basically the meat of flash content verification.

[bjoernQ]

There is ESP_FLASH_VERIFY_MD5 command ( https://github.com/espressif/esptool/blob/8538b8983e5e04ad5d6f63620d609bfa63412eb3/flasher_stub/include/stub_flasher.h#L53 )

I think it's not yet supported by the Rust esp-flasherstub but by the esp-tool stub and in ROM (not for ESP-8266)

[bjoernQ]

Started to look into ESP_FLASH_VERIFY_MD5 ... as mentioned, we don't have it supported by the Rust esp-flasherstub which is not that good

Tried without a flasher stub and it needs quite a few changes to espflash internals because of the way we currently parse responses into CommandResponse (and how we currently cannot return something that is not u32)

Additionally, there are at least two different response formats for that command