gen-demo-header.py: Fix running without git

gen-demo-header.py: Fix running without git

The script assumes that the git program is in the PATH which may not be always
the case, so if git was not found, fallback to the former behavior of making
VCS_TAG an empty string if the profile option was not set to 'devel'.

Also fix the call to the script so that we do indeed call GIT on the source GIT
repo so that we can get the right short SHA1 sum.

Re-organize the code a bit in the process, so that things seem cleaner.