Only write to FLASH once 64 bytes are buffered

[SteveCEvans]

Black box data to be written to FLASH is buffered in a 128 byte buffer, but is being written more often than necessary, typically 28 bytes at a time. This PR changes that to write no less than 64 bytes.

This reduces CPU load as fewer write operations are required.

[blckmn]

AUTOMERGE: (FAIL)

• github identifies PR as mergeable -> FAIL
• assigned to a milestone -> PASS
• cooling off period lapsed -> FAIL
• commit count less or equal to three -> PASS
• Don't merge label NOT found -> PASS
• at least one RN: label found -> FAIL
• Tested label found -> FAIL
• assigned to an approver -> PASS
• approver count at least three -> FAIL

[hydra]

@SteveCEvans It has been a preference to avoid boolean arguments in favor or two methods, one that behaves as though 'true' was specified, and one that behaves as if 'false' was specified. This leads to greater readability as you always have to look up what the 'true' or false does. A private (static) 'inner' method that uses a boolean argument is better, as the public api wouldn't contain the inner method.

i.e.

public:

flashfsFlushAsync(void)
flashfsForceFlushAsync(void)

private:

flashfsFlushAsyncInner(bool force)

another preferred approach is to use an enum instead of the bool so that the argument is named.

flashfsFlushAsync(FLUSH_FORCE);
flashfsFlushAsync(FLUSH_NORMAL);