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/entropy_poll.c b/library/entropy_poll.c
index 4556f88..413be4a 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -106,7 +106,7 @@
     /* MemSan cannot understand that the syscall writes to the buffer */
 #if defined(__has_feature)
 #if __has_feature(memory_sanitizer)
-    memset( buf, 0, buflen );
+    mbedtls_platform_memset( buf, 0, buflen );
 #endif
 #endif
     return( syscall( SYS_getrandom, buf, buflen, flags ) );
@@ -218,7 +218,7 @@
     size_t use_len = MBEDTLS_ENTROPY_BLOCK_SIZE;
     ((void) data);
 
-    memset( buf, 0, MBEDTLS_ENTROPY_BLOCK_SIZE );
+    mbedtls_platform_memset( buf, 0, MBEDTLS_ENTROPY_BLOCK_SIZE );
 
     if( mbedtls_nv_seed_read( buf, MBEDTLS_ENTROPY_BLOCK_SIZE ) < 0 )
       return( MBEDTLS_ERR_ENTROPY_SOURCE_FAILED );