[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/include/gpxe/pci_ids.h

@@ -7,6 +7,8 @@
  *	Please keep sorted.
  */
 
+FILE_LICENCE ( GPL2_ONLY );
+
 /* Device classes and subclasses */
 
 #define PCI_CLASS_NOT_DEFINED		0x0000