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/md.c b/library/md.c
index 882942e..92fe3a4 100644
--- a/library/md.c
+++ b/library/md.c
@@ -387,7 +387,7 @@
 
 void mbedtls_md_init( mbedtls_md_context_t *ctx )
 {
-    memset( ctx, 0, sizeof( mbedtls_md_context_t ) );
+    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_md_context_t ) );
 
 #if defined(MBEDTLS_MD_SINGLE_HASH)
     mbedtls_md_info_init( mbedtls_md_get_handle( ctx ),
@@ -563,8 +563,8 @@
     opad = (unsigned char *) ctx->hmac_ctx +
         mbedtls_md_info_block_size( md_info );
 
-    memset( ipad, 0x36, mbedtls_md_info_block_size( md_info ) );
-    memset( opad, 0x5C, mbedtls_md_info_block_size( md_info ) );
+    mbedtls_platform_memset( ipad, 0x36, mbedtls_md_info_block_size( md_info ) );
+    mbedtls_platform_memset( opad, 0x5C, mbedtls_md_info_block_size( md_info ) );
 
     for( i = 0; i < keylen; i++ )
     {