S368275: don't mess with the watchdog on darwin
Summary:
Messing with a stopped watchdog starts it.
Can do something nicer later.
Test Plan:
```
0 ~/local/openbmc/tools/flashy $ go test ./...
? github.com/facebook/openbmc/tools/flashy/flash_procedure [no test files]
? github.com/facebook/openbmc/tools/flashy/lib/logger [no test files]
? github.com/facebook/openbmc/tools/flashy/tests [no test files]
? github.com/facebook/openbmc/tools/flashy/utilities [no test files]
ok github.com/facebook/openbmc/tools/flashy 3.350s
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/bletchley 0.008s
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/common 0.296s
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/galaxy100 0.008s
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/wedge100 0.012s
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/yamp 0.009s
ok github.com/facebook/openbmc/tools/flashy/install 0.009s
ok github.com/facebook/openbmc/tools/flashy/lib/fileutils 0.014s
ok github.com/facebook/openbmc/tools/flashy/lib/flash 0.011s
ok github.com/facebook/openbmc/tools/flashy/lib/flash/flashcp 0.011s
ok github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils 0.008s
ok github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils/devices 0.009s
ok github.com/facebook/openbmc/tools/flashy/lib/step 0.012s
ok github.com/facebook/openbmc/tools/flashy/lib/utils 0.455s
ok github.com/facebook/openbmc/tools/flashy/lib/validate 0.009s
ok github.com/facebook/openbmc/tools/flashy/lib/validate/image 0.014s
ok github.com/facebook/openbmc/tools/flashy/lib/validate/partition 0.037s
0 ~/local/openbmc/tools/flashy $ ./build.sh && ./build_dev.sh
0 ~/local/openbmc/tools/flashy $
```
```
$ oobgrader --host rsw029-oob.p076.f01.rva3.tfbnw.net --primary-only --wait --flashy-tag 734df50 --force --allow-downgrade
```
-> https://fburl.com/scuba/openbmc_upgrades/7e89e86b
Console messages during upgrade (no mention of watchdog):
```
[ 2783.001706] 11_drop_caches (6255): drop_caches: 3
[ 2873.352788] reboot: Restarting system
U-Boot 2019.04 fbdarwin-v2022.27.1 (Jul 06 2022 - 20:26:37 +0000)
```
Reviewed By: kawmarco
Differential Revision: D49734788
fbshipit-source-id: 1c2ce62b9d94f7fe8eb66a1969b0a2fc1d0c19a9
