build: allow rootfs builds w/o git or VERSION file deps

We set the VERSION variable consistently across Makefiles to
'unknown'  if the file is empty or not present.
We also use git commands consistently for calculating the COMMIT,
COMMIT_NO variables, not erroring out when building outside of
a git repository.
In create_summary_file we also account for a missing/empty VERSION
file.
This makes e.g. the UVM build process in an environment where we
build outside of git with a minimal/reduced set of files smoother.

Signed-off-by: Manuel Huber <mahuber@microsoft.com>

tools/osbuilder/scripts/lib.sh

@@ -181,7 +181,8 @@ create_summary_file()
 	[ "$AGENT_INIT" = yes ] && agent="${init}"
 
 	local -r agentdir="${script_dir}/../../../"
-	local -r agent_version=$(cat ${agentdir}/VERSION)
+	local agent_version=$(cat ${agentdir}/VERSION 2> /dev/null)
+	[ -z "$agent_version" ] && agent_version="unknown"
 
 	cat >"$file"<<-EOF
 	---