nand-part: create one nand-part program to handle both A10 and A20

Build one nand-part program to handle both A10 and A20 mbr header formats.
Changed -f option to take an "a10" or "a20" argument to specify which mbr
format to force.

Signed-off-by: Patrick Wood <patrickhwood@gmail.com>
nand-part: added/updated open source copyright headers

Signed-off-by: Patrick Wood <patrickhwood@gmail.com>
nand-part: updated usage message

Signed-off-by: Patrick Wood <patrickhwood@gmail.com>
nand-part: fixed error in -f usage message

Signed-off-by: Patrick Wood <patrickhwood@gmail.com>