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/hmac_drbg.c b/library/hmac_drbg.c
index b51e9b1..e00a834 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -56,7 +56,7 @@
  */
 void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx )
 {
-    memset( ctx, 0, sizeof( mbedtls_hmac_drbg_context ) );
+    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_hmac_drbg_context ) );
 
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_init( &ctx->mutex );
@@ -141,7 +141,7 @@
     if( ( ret = mbedtls_md_hmac_starts( &ctx->md_ctx, ctx->V,
                                         mbedtls_md_get_size( md_info ) ) ) != 0 )
         return( ret );
-    memset( ctx->V, 0x01, mbedtls_md_get_size( md_info ) );
+    mbedtls_platform_memset( ctx->V, 0x01, mbedtls_md_get_size( md_info ) );
 
     if( ( ret = mbedtls_hmac_drbg_update_ret( ctx, data, data_len ) ) != 0 )
         return( ret );
@@ -178,7 +178,7 @@
         }
     }
 
-    memset( seed, 0, MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT );
+    mbedtls_platform_memset( seed, 0, MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT );
 
     /* IV. Gather entropy_len bytes of entropy for the seed */
     if( ( ret = ctx->f_entropy( ctx->p_entropy,
@@ -268,7 +268,7 @@
      */
     if( ( ret = mbedtls_md_hmac_starts( &ctx->md_ctx, ctx->V, md_size ) ) != 0 )
         return( ret );
-    memset( ctx->V, 0x01, md_size );
+    mbedtls_platform_memset( ctx->V, 0x01, md_size );
 
     ctx->f_entropy = f_entropy;
     ctx->p_entropy = p_entropy;