sleep briefly while flashing
Summary:
Go idle periodically when flashing the image, to allow other work
on the system to happen, for example SPI I/O. For a 32MB flash
image, this would result in an extra 8s of idle time during wipe plus
8s during flashing. For S434121.
Test Plan:
```
0 ~/local/openbmc/tools/flashy $ ./build.sh && ./build_dev.sh && 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 2.024s
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/bletchley (cached)
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/common (cached)
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/galaxy100 (cached)
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/grandteton (cached)
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/wedge100 (cached)
ok github.com/facebook/openbmc/tools/flashy/checks_and_remediations/yamp (cached)
ok github.com/facebook/openbmc/tools/flashy/install 0.007s
ok github.com/facebook/openbmc/tools/flashy/lib/fileutils (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/flash 0.007s
ok github.com/facebook/openbmc/tools/flashy/lib/flash/flashcp 0.508s
ok github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils/devices (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/step (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/utils (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/validate (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/validate/image (cached)
ok github.com/facebook/openbmc/tools/flashy/lib/validate/partition (cached)
0 ~/local/openbmc/tools/flashy $
```
Build ephemeral fbpkg and flash a wedge100 using it:
```
Host Workflow ID Progress Status Result
---------------------------------- ------------------------------------ ---------- ----------------------- ----------------------
fboss8382003-oob.snc1.facebook.com b4600065-e674-4696-9378-5c313ae45819 finished WorkflowStatus.FINISHED FinishStatus.SUCCEEDED
```
-> https://fburl.com/scuba/openbmc_upgrades/yksvon8b
Reviewed By: kawmarco
Differential Revision: D59927810
fbshipit-source-id: 7db5bb534575d360816a2ec27bea361936fe16dd
