* Fix Updater potential overflow, add host tests
Fixes#4674
The Updater class could, when exactly 4K bytes were in the buffer but
not yet written to flash, allow overwriting data written to it beyond
the passed-in size parameter.
Fix per @jason-but's suggestion, and add a host test (plus minor changes
to Updater code to support host testing).
* Add missed mock file
* Remove most testing ifdefs fro updater
Per @mcspr's suggestion, we can pass in fake link symbols allowing
Updater to take the address of `_FS_start`/etc. even when building on
the host for testing.
There is still a single remaining wifi_set_power_mode ifdef'd and a
duplication of the digitalWrite/pinMode for testing vs. host building.
Co-authored-by: Develo <deveyes@gmail.com>
