How to Combine Insert, Update, and Delete in the same `Instead Of` Trigger, in Oracle db?
From Coding the Trigger Body 


  Detecting the DML Operation that Fired a Trigger
  
  If more than one type of DML operation can fire a trigger (for
  example, ON INSERT OR DELETE OR UPDATE OF emp), the trigger body can
  use the conditional predicates INSERTING, DELETING, and UPDATING to
  check which type of statement fire the trigger.
  
  Within the code of the trigger body, you can execute blocks of code
  depending on the kind of DML operation that fired the trigger:

IF INSERTING THEN ... END IF;
IF UPDATING THEN ... END IF;
IF DELETING THEN ... END IF;



in your case 

...
if inserting then

INSERT INTO SRC_TBL (CFG_NAME, CFG_VALUE) 
         VALUES(:NEW.CFG_NAME, :NEW.CFG_VALUE);

elsif updating then

 UPDATE SRC_TBL SET CFG_VALUE = :NEW.CFG_VALUE
  WHERE CFG_NAME = :NEW.CFG_NAME;

elsif deleting then

 ...

END IF;