flashy-849f885

handle flashy validation for yosemite5a7/AST2700

Summary:
Currently any attempt to run oobgrader against a ysoemite5a7 device is failing as it cannot verify the image in flashy checks and remediations

Let's add a fix to the whole image for the U-Boot version string (step 40) and adding the a7's 4088 KB manifest offset to the candidate list (step 41)

Test Plan:
```
$oobgrader --host rtptest6715-oob.snc8.facebook.com --fbpkg-tag openbmc.image.yosemite5a7:v2026.27.1 --flashy-tag 5d49d87 --wait
```
Created job: https://fburl.com/scuba/openbmc_upgrades/b81b19sc

BMC successfully upgraded:
```
$ ssh root@rtptest6715-oob.snc8.facebook.com
Last login: Mon Jul 27 12:11:30 2026 from 2401:db00:eef0:1120:3520:0:a804:7c6
root@bmc:~# cat /etc/issue
OpenBMC Release yosemite5a7-v2026.27.1

```

Reviewed By: amithash, ryazwinski, williamspatrick

Differential Revision: D112993974

fbshipit-source-id: cba093b85dcef6961ed8bafe4528846e9f27c87f