[legal] Add a selection of FILE_LICENCE declarations

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

src/drivers/net/3c90x.h

@@ -45,6 +45,8 @@
  * indent options: indent -kr -i8 3c90x.c
  */
 
+FILE_LICENCE ( BSD2 );
+
 #ifndef __3C90X_H_
 #define __3C90X_H_