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/pem.c b/library/pem.c
index 897c8a0..96e4718 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -48,7 +48,7 @@
 #if defined(MBEDTLS_PEM_PARSE_C)
 void mbedtls_pem_init( mbedtls_pem_context *ctx )
 {
-    memset( ctx, 0, sizeof( mbedtls_pem_context ) );
+    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_pem_context ) );
 }
 
 #if defined(MBEDTLS_MD5_C) && defined(MBEDTLS_CIPHER_MODE_CBC) &&         \
@@ -61,7 +61,7 @@
 {
     size_t i, j, k;
 
-    memset( iv, 0, iv_len );
+    mbedtls_platform_memset( iv, 0, iv_len );
 
     for( i = 0; i < iv_len * 2; i++, s++ )
     {