[crypto] Allow for addition of arbitrary Weierstrass curve points

ECDSA verification requires the ability to add two arbitrary curve
points (as well as the ability to multiply a curve point by a scalar).

Add an elliptic curve method to perform arbitrary point addition.
Pass in curve points as affine coordinates: this will require some
redundant conversions between affine coorfinates and the internal
representation as projective coordinates in Montgomery form, but keeps
the API as simple as possible.  Since we do not expect to perform a
high volume of ECDSA signature verifications, these redundant
calculations are an acceptable cost for keeping the code simple.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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