The Windows version of mkdir() does not share the same signature
as the POSIX version. Add rk_mkdir() with an POSIX compliant signature
to libroken. The current implementation ignores the 'mode' parameter
on Windows but a future implementation could provide the required
functionality with native Win32 APIs.
Change-Id: I084d42e0f27932e9b8131d310dbf34849f5fe4b6
