BMC dual image support for static layout

[Lei Yu]

The dual image support for static layout is implemented and submitted to gerrit:
https://gerrit.openbmc-project.xyz/q/topic:bmc-dual-image

The related doc is on gerrit as well:
https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/36407

The implementation basically adds:
1. A config option `bmc-static-dual-image` to define the
`BMC_STATIC_DUAL_IMAGE` macro.
2. The function to process the alt-bmc and create the related DBus objects.
3. The aspeed-specific code to tell which flash it is running.
4. A yaml interface is added to specify the code update target
5. The code-update support to flash the primary or secondary or both flashes.
6. The sync between two flashes.

Be noted that the images could be the same on the both flashes, so it
appends a salt to the version string to generate different version
IDs, and that change should impact the UBI layout.

@Adriana Kobylak please help to review the code and let's see how to
make it work for UBI/emmc case.

-- 
BRs,
Lei YU