RSA requires modular exponentiation using arbitrarily large integers.
Given the sizes of the modulus and exponent, all required calculations
can be done without any further dynamic storage allocation.  The x86
architecture allows for efficient large integer support via inline
assembly using the instructions that take advantage of the carry flag
(e.g. "adcl", "rcrl").

This implemention is approximately 80% smaller than the (more generic)
AXTLS implementation.

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