fix(ota): Add legacy option for devices using MD5 authentication

[lucasssvaz]

Description of Change

This pull request updates the espota.py tool to add support for authenticating with devices that use an older, insecure MD5-based challenge/response protocol, improving compatibility with legacy firmware. The main changes include introducing a new --md5-target option, updating the authentication flow to handle both MD5 and SHA256 mechanisms, and adjusting the argument parsing and function signatures accordingly.

This argument should be necessary only once when upgrading from an image with the older authentication method to the new one.

Authentication protocol updates:

• Added a new --md5-target (-m) command-line flag to indicate that the target device uses MD5-based authentication, for compatibility with old firmware.
• Modified the serve function and its call sites to accept an additional md5_target argument, controlling which authentication protocol is used. [1] [2]
• Updated the authentication logic in serve to:
  • Receive the correct nonce length depending on protocol (37 bytes for MD5, 69 for SHA256).
  • Generate the client nonce and challenge/response using MD5 or SHA256 as appropriate.
  • Expect the correct response length from the device (32 bytes for MD5, 64 for SHA256).

Test Scenarios

Tested with ESP32-C6

Related links

Closes #11855

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