polarssl: replace BIGNUM_C_IMPLEMENTATION check

Replace the BIGNUM_C_IMPLEMENTATION check by a check on POLARSSL_HAVE_ASM
and __arm__. This way it's possible to use this code on non arm, but
also to run the GNU/Linux emulation on an arm machine, which is useful
for debugging and profiling.

src/Makefile

@@ -34,7 +34,7 @@ USE_EVENTFLAG = yes
 ifeq ($(EMULATION),)
 DEFS += -DFLASH_UPGRADE_SUPPORT
 else
-DEFS += -DBN256_C_IMPLEMENTATION -DBIGNUM_C_IMPLEMENTATION
+DEFS += -DBN256_C_IMPLEMENTATION
 endif
 
 ifneq ($(ENABLE_DEBUG),)