Add support for compressed format to ecp_point_read_binary

[meh]

Since it's supported when writing it should also be supported when reading.

I sent a corporate signed CLA earlier.

[RonEld]

resolves #1616

@meh Thank you for your contribution! please fix CI failure:

The following tests FAILED:
	 28 - ecp-suite (Failed)
	 51 - pkparse-suite (Failed)

[mpg]

Hi @meh, and thanks for you contribution.

Unfortunately, this implementation does not conform to the standard, as way more computations are required. (See #521 for something more realistic.) Also, it doesn't come with any tests (which would have caught the issues).

[mpg]

This is not correct. The Y coordinate should be computed by using the curve equation to computer Y^2, then extracting a square root modulo P, and only then using that bit to select which of the two square roots we want.

[mpg]

Support for compressed format has been deprecated by RFC 8422 in the context of TLS, which reflects a more general sentiment in the ECC community to prefer uncompressed format. Also, implementing it correctly for all supported curves would require substantial code, impacting our footprint.

At this point, we're unlikely to want to add that amount of code for a feature that's formally deprecated in TLS and being abandoned more generally, so I'm closing this PR.

Thanks for your contribution and interest in Mbed TLS anyway.