- Never return from 'nand_do_upgrade', not even in case of errors, as that
would cause execution of sysupgrade code not intended for NAND devices.
- Unify handling of sysupgrade success and failure.
- Detect and report more error conditions.
- Fix outdated/incorrect/unclear comments.
Signed-off-by: Rodrigo Balerdi <lanchon@gmail.com>