Bignum: Apply naming conventions

Numbers:

- A, B for mbedtls_mpi_uint* operands
- a, b for mbedtls_mpi_uint operands
- X or x for result
- HAC references where applicable

Lengths:

- Reserve size or length for length/size in bytes or byte buffers.
- For length of mbedtls_mpi_uint* buffers use limbs
- Length parameters are qualified if possible (eg. input_length or
  a_limbs)

Setup functions:

- The parameters match the corresponding structure member's name
- The structure to set up is a standard lower case name even if in other
  functions different naming conventions would apply

Scope of changes/conventions:

- bignum_core
- bignum_mod
- bignum_mod_raw

Signed-off-by: Janos Follath <janos.follath@arm.com>

library/constant_time.c

@@ -744,9 +744,9 @@ cleanup:
 /*
  * Compare unsigned values in constant time
  */
-unsigned mbedtls_mpi_core_lt_ct( const mbedtls_mpi_uint *X,
-                                 const mbedtls_mpi_uint *Y,
-                                 size_t len )
+unsigned mbedtls_mpi_core_lt_ct( const mbedtls_mpi_uint *A,
+                                 const mbedtls_mpi_uint *B,
+                                 size_t limbs )
 {
     unsigned ret, cond, done;
 
@@ -754,31 +754,31 @@ unsigned mbedtls_mpi_core_lt_ct( const mbedtls_mpi_uint *X,
      * their scope. */
     ret = cond = done = 0;
 
-    for( size_t i = len; i > 0; i-- )
+    for( size_t i = limbs; i > 0; i-- )
     {
         /*
-         * If Y[i - 1] < X[i - 1] then X < Y is false and the result must
+         * If B[i - 1] < A[i - 1] then A < B is false and the result must
          * remain 0.
          *
          * Again even if we can make a decision, we just mark the result and
          * the fact that we are done and continue looping.
          */
-        cond = mbedtls_ct_mpi_uint_lt( Y[i - 1], X[i - 1] );
+        cond = mbedtls_ct_mpi_uint_lt( B[i - 1], A[i - 1] );
         done |= cond;
 
         /*
-         * If X[i - 1] < Y[i - 1] then X < Y is true.
+         * If A[i - 1] < B[i - 1] then A < B is true.
          *
          * Again even if we can make a decision, we just mark the result and
          * the fact that we are done and continue looping.
          */
-        cond = mbedtls_ct_mpi_uint_lt( X[i - 1], Y[i - 1] );
+        cond = mbedtls_ct_mpi_uint_lt( A[i - 1], B[i - 1] );
         ret |= cond & ( 1 - done );
         done |= cond;
     }
 
     /*
-     * If all the limbs were equal, then the numbers are equal, X < Y is false
+     * If all the limbs were equal, then the numbers are equal, A < B is false
      * and leaving the result 0 is correct.
      */