Refactor flash class, support unrecognized chips

[kuba2k2]

• the BK7231Serial class is refactored
• it first checks if it knows the bootloader CRC, otherwise it tries to guess if the chip is BK7231N (whether end offset of CRC is inclusive or not)
• it can also detect Flash Size if not known in advance (by checking where the bootloader "modulo" wraparound is)
• it should fix most issues on BK7231N chips with new/different keys, as well as issues on BK7252 with flash bigger than 2MiB