[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.
diff --git a/src/drivers/nvs/spi.c b/src/drivers/nvs/spi.c
index dbdc32f..793080a 100644
--- a/src/drivers/nvs/spi.c
+++ b/src/drivers/nvs/spi.c
@@ -16,6 +16,8 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <stddef.h>
 #include <errno.h>
 #include <unistd.h>