Static layout PNOR support in openpower-pnor-code-mgmt

[Lei YU]

As a follow up of [VPNOR discussion][1], the static layout PNOR
support in openpower-pnor-code-mgmt is implemented and pushed to gerrit
starting from https://gerrit.openbmc-project.xyz/#/c/openbmc/openpower-pnor-code-mgmt/+/18747/

Code reviews and tests are welcome.

To test the feature:
1. Pick the above gerrit commits and build with --disable-ubifs_layout
2. Generate pnor in static.tar.gz format, by below command:
    ./generate-squashfs <xxx.pnor>   # xxx.pnor.static.tar.gz will be generated
   This tar.gz will be used to do the code update.
3. Do code update, gard reset, factory reset the same as VPNOR, described in
   [host-code-update.md][2] and [REST-cheatsheet.md][3]

I only tested it on Romulus, and will test it on Palmetto. Hopefully it will
be tested on other OpenPOWER machines before the patches are merged.

Note: image verification feature is not implemented yet.

[1]: https://lists.ozlabs.org/pipermail/openbmc/2019-February/015104.html
[2]: https://github.com/openbmc/docs/blob/master/code-update/host-code-update.md
[3]: https://github.com/openbmc/docs/blob/master/REST-cheatsheet.md