Fix Change-Id generator helper

On certain platforms (at least alpine) the commonly used

 echo "I$(uuidgen | openssl dgst -sha1 -binary | xxd -p)"

could create a string with trailing whitespace. Replacing that with

 echo "I$(uuidgen | sha1sum | awk '{print $1}')"

does not have that problem and what it does is more obvious at a glance.

Change-Id: I2f4b490272a19d123a1dca2ba13f8b222e9c3035