tools: fw-update: allow ISO 8610 date formats

Currently, the ReleaseDateTime data is expected to be in a
"%d/%m/%Y %H:%M:%S" format.

However, this is ambiguous (ie., is locale-specific).

This change adds supoort to allow an ISO 8601 format of:

 "%Y-%m-%dT%H:%M:%S"

and a relaxed version with the 'T' separator replaced with a space:

 "%Y-%m-%d %H:%M:%S"

Update the readme and example json to switch to the preferred format.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Change-Id: Ica3ae824979df6b666b8abb619ceaedffd90b374