build: Use OPNFV_GIT_SHA for ISO preparer ID

build: Use OPNFV_GIT_SHA for ISO preparer ID

isoinfo -i lists the following information for Fuel@OPNFV ISO:
"Data preparer id: 86aafaf5454a846c417848bb94f264c4420160f3"
where the SHA hash is Fuel git repo HEAD SHA.

For Armband's build system, using only the Fuel commit ID is not
enough to fully describe the state of the source code, as patches
are also applied to other fuel modules (as git submodules).
Instead, a pointer to a valid Armband commit ID should be used.

However, Armband overrides OPNFV_GIT_SHA to the Armband git repo
commit hash, so the ISO metadata should also reflect this.

While at it, allow product name to be overriden, to signal the
commit ID should be looked up inside the Armband repository.

Change-Id: I33ad490f1afe28c1d439dda40e39cee1955e0ac2
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>