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/ecp.c b/library/ecp.c
index db36191..5c2e0f1 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1553,7 +1553,7 @@
     size_t i, j;
     unsigned char c, cc, adjust;
 
-    memset( x, 0, d+1 );
+    mbedtls_platform_memset( x, 0, d+1 );
 
     /* First get the classical comb values (except for x_d = 0) */
     for( i = 0; i < d; i++ )