Replace memset() with mbedtls_platform_memset()

Steps:

1. sed -i 's/\bmemset(\([^)]\)/mbedtls_platform_memset(\1/g' library/*.c tinycrypt/*.c include/mbedtls/*.h scripts/data_files/*.fmt

2. Manually edit library/platform_util.c to revert to memset() in the
implementations of mbedtls_platform_memset() and mbedtls_platform_memcpy()

3. egrep -n '\<memset\>' library/*.c include/mbedtls/*.h tinycrypt/*.c
The remaining occurrences are in three categories:
    a. From point 2 above.
    b. In comments.
    c. In the initialisation of memset_func, to be changed in a future commit.
diff --git a/library/pkcs12.c b/library/pkcs12.c
index e16d0a9..85eb31e 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -99,8 +99,8 @@
     if( pwdlen > PKCS12_MAX_PWDLEN )
         return( MBEDTLS_ERR_PKCS12_BAD_INPUT_DATA );
 
-    memset( &salt, 0, sizeof(mbedtls_asn1_buf) );
-    memset( &unipwd, 0, sizeof(unipwd) );
+    mbedtls_platform_memset( &salt, 0, sizeof(mbedtls_asn1_buf) );
+    mbedtls_platform_memset( &unipwd, 0, sizeof(unipwd) );
 
     if( ( ret = pkcs12_parse_pbe_params( pbe_params, &salt,
                                          &iterations ) ) != 0 )
@@ -283,7 +283,7 @@
     else
         v = 128;
 
-    memset( diversifier, (unsigned char) id, v );
+    mbedtls_platform_memset( diversifier, (unsigned char) id, v );
 
     pkcs12_fill_buffer( salt_block, v, salt, saltlen );
     pkcs12_fill_buffer( pwd_block,  v, pwd,  pwdlen  );