[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/pnic_api.h

@@ -12,6 +12,8 @@
  * time from PNIC_REG_DATA.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /*
  * PCI parameters
  */