tls: P256: propagate constants, create dedicated "subtract p256_mod" function

8 instances of this subtraction probably warrant a few bytes more of code.

function                                             old     new   delta
sp_256_sub_8_p256_mod                                  -      71     +71
sp_256_mont_sub_8                                      -      29     +29
sp_256_mont_dbl_8                                      -      26     +26
sp_256_mont_reduce_8                                 262     257      -5
sp_256_ecc_mulmod_8                                 1171    1161     -10
sp_256_proj_point_dbl_8                              374     359     -15
static.sp_256_mont_sub_8                              29       -     -29
static.sp_256_mont_dbl_8                              31       -     -31
------------------------------------------------------------------------------
(add/remove: 3/2 grow/shrink: 0/3 up/down: 126/-90)            Total: 36 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed