libtommath: remove unnecessary comparison

As pointed out by Steffen Jaeckel [https://github.com/sjaeckel],
within bn_mp_prime_next_prime() t <= PRIME_SIZE as per the check
at the top of the function.  Remove the unnecessary comparison in
a for loop conditional.

Change-Id: I868bee1a7a019e0ab06bf2b81cc71cf66ca9acff
libtommath: protect functions by tommath_class macros

libtommath protects the inclusion of functions by wrapping their
declarations by CPP #ifdef tests and defining the matching macros
within libtommath_class.h.

Add missing macros:

  BN_MP_FIND_PRIME_C
  BN_MP_ISPRIME_C

Change-Id: Ic577300114f8e88d7a3af3d9f717a249d57f03b1