[crypto] Rename bigint_rol()/bigint_ror() to bigint_shl()/bigint_shr()

[crypto] Rename bigint_rol()/bigint_ror() to bigint_shl()/bigint_shr()

The big integer shift operations are misleadingly described as
rotations since the original x86 implementations are essentially
trivial loops around the relevant rotate-through-carry instruction.

The overall operation performed is a shift rather than a rotation.
Update the function names and descriptions to reflect this.

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