Add nand_fileio_* helper APIs.

Add nand_fileio_* helper APIs.

This patch provides helpers APIs that will eliminate duplicated code in
the the NAND 'dump' and 'write' commands by factoring their common code
into static helper functions.  These helpers may be useful for creating
new commands, as shown in the final patch to 'verify' flash from a file.

Several previously unreported error conditions now generate messages and
propogate the return codes, such as when the file fails to open and bad
arguments are given.  These changes will fix a possible memory leak in
nand dump command, in the case that the dump file failed to open.

Overall, the changes should be functionally equivalent, but the
resulting code will be easier to improve and extend consistently.