fcm make: extract: improve primary location check

Improve primary location re-set checking on inheritance. Compare old
value with normalised version of new value. E.g. use of location
keywords and extra slashes at the end of the path do not result in a
configuration conflict.

(Also remove an unused template string from error report.)
No, because `FCM_CONF_PATH` in the previous line overrides the default search path for site and user configuration.