Rename plain c option and update comments

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/aesce.c b/library/aesce.c
index 4b7e048..8aa0789 100644
--- a/library/aesce.c
+++ b/library/aesce.c
@@ -99,7 +99,7 @@
 #include <sys/auxv.h>
 #endif
 
-#if !defined(MBEDTLS_AES_DONT_USE_SOFTWARE_CRYPTO)
+#if !defined(MBEDTLS_AES_USE_HARDWARE_ONLY)
 /*
  * AES instruction support detection routine
  */