ecdsa: Allow ECDSA signatures to be actual length

ECDSA signatures are variable length.  They are also encoded as ASN.1.
The ASN.1 parser we use is given the length, and will return a decoding
error if the signature block is not sufficiently long.  Instead of
requiring the signature block be padded to the longest possible length a
signature can be, allow them to be their natural length.

This allows image signing tools to be able to generate signatures that
don't have this padding.  Along with removing the pad removal code from
the EC224 code, this will allow this code to correctly validate all
signatures, not just 255 out of 256.

Signed-off-by: David Brown <david.brown@linaro.org>