fix(spi_attach): Unattach previously attached SPI flash (ESPTOOL-847)

[radimkarnis]

Fixed --spi-connection not working correctly in esp-stub-flasher (original esptool.py commit: espressif/esptool@afaa7d2)

• Stub flasher calls spi_flash_attach automatically during boot-up, while ROM does this only when SPI_ATTACH command is sent by esptool. This means, that stub flasher essentially tries to attach a flash chip twice if --spi-connection is used.
• It is needed to "unattach" the flash chip before attaching another one. This is now done by reconfiguring the SPI pins in the IO MUX back to function as standard GPIOs.
• This fixes the failing test_short_flash_to_external_stub in esptool test suite.

[jessebraham]

That new lint rule is a bit annoying, you can just append this to the end of Cargo.toml to silence that:

[lints.rust]
unexpected_cfgs = "allow"

[dobairoland]

LGTM. Thanks @radimkarnis.

[jessebraham]

LGTM, thanks!

[radimkarnis]

@jessebraham @dobairoland thanks for the quick reviews!