FD_OFFSET

[Sergei Sharonov]

Suggest moving FD_OFFSET into uffs_config.h

Rationale:

UFFS may not be the only source of FDs in the system. Need some mechanism to reserve FDs for use by other drivers. UFFS already does this for stdio via FD_OFFSET. Moving it into user configurable header will let user increase pool of reserved FDs when needed. 

I also added the following macro to validate it:

#if (1 << FD_SIGNATURE_SHIFT) < (MAX_OBJECT_HANDLE + FD_OFFSET)

#error Not enough shift

#endif

Does this look correct?

Regards,

Sergei