Rename dds/src/xtypes/builtin.c to xtypes_builtin.c

This change is made to avoid conflicts between the
symbols in the NuttX code (contains a builtin) and
the symbols from the DDS code.
Fix rand and srand calls for nuttx
Quick fix for mbtowc in NuttX.

Ideally the funtion should be included in the DDS tree.
Take a look at https://github.com/lattera/glibc/blob/master/stdlib/mbtowc.c
for a reference implementation.