bigint_mod_multiply() and bigint_mod_exp() require a fixed amount of
temporary storage for intermediate results.  (The amount of temporary
storage required depends upon the size of the integers involved.)

When performing calculations for 4096-bit RSA the amount of temporary
storage space required will exceed 2.5kB, which is too much to
allocate on the stack.  Avoid this problem by forcing the caller to
allocate temporary storage.

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

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>