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
diff --git a/tools/fw-update/metadata-example.json b/tools/fw-update/metadata-example.json
index ac080af..f6d65d8 100644
--- a/tools/fw-update/metadata-example.json
+++ b/tools/fw-update/metadata-example.json
@@ -2,7 +2,7 @@
     "PackageHeaderInformation": {
         "PackageHeaderIdentifier": "F018878CCB7D49439800A02F059ACA02",
         "PackageHeaderFormatVersion": 1,
-        "PackageReleaseDateTime": "25/12/2021 00:00:00",
+        "PackageReleaseDateTime": "2021-12-25 00:00:00",
         "PackageVersionString": "VersionString1"
     },
     "FirmwareDeviceIdentificationArea": [