[crypto] Add a generic implementation of a Montgomery ladder

[crypto] Add a generic implementation of a Montgomery ladder

The Montgomery ladder may be used to perform any operation that is
isomorphic to exponentiation, i.e. to compute the result

    r = g^e = g * g * g * g * .... * g

for an arbitrary commutative operation "*", base or generator "g", and
exponent "e".

Implement a generic Montgomery ladder for use by both modular
exponentiation and elliptic curve point multiplication (both of which
are isomorphic to exponentiation).

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