This commit adds tools/gencromfs.c, a tool that can be used to generate CROMFS file system images.

Squashed commit of the following:

    tools/gencromfs.c:  Seems basically functional but has not has not yet been integrated with CROMFS.
    tools/gencromfs.c:  Revist previous commit.  Let's make no assumptions about the compatibility of the host system and NuttX.
    tools/gencromfs.c:  I think access mode bits are sufficiently standard that we can just copy them from the host.  This is don't mostly to pick up the executable bits and other conbinations of bits without doing a bit-for-bit decode.
    tools/gencromfs.c:  Code complete but only partially functional.
    tools/gencromfs.c:  type definitions must precede data definitions
    tools/gencromfs.c:  Adds a little more logic to gencromfs tools.  Still not complete but can traverse directories and generate directory and hard-link nodes.
    tools/gencromfs.c: Beginning of a genromfs tool that will eventually be used for generation of CROMFS file system images.