backports: catch make in backport template directory

Catch running make in the backport template directory by
checking for the .local-symbols file. If it isn't there
then we're in the input/template directory rather than in
an output tree, give the user a hint about it instead of
keeling over.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>