imgtool: Fix verify command for edcsa-p384 signed images

Fixed hash algorithm defaults to SHA256 in case no key provided.
Verification improved by adding check for key - tlv mismatch,
VerifyResult.KEY_MISMATCH added to indicate this case.
Multiple styling fixes and import optimisation, exception handling.

Signed-off-by: Rustam Ismayilov <rustam.ismayilov@arm.com>
Change-Id: I61a588de5b39678707c0179f4edaa411ceb67c8e