[crypto] Eliminate temporary working space for bigint_mod_invert()

[crypto] Eliminate temporary working space for bigint_mod_invert()

With a slight modification to the algorithm to ignore bits of the
residue that can never contribute to the result, it is possible to
reuse the as-yet uncalculated portions of the inverse to hold the
residue.  This removes the requirement for additional temporary
working space.

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