[build] Mark core files as permitted for UEFI Secure Boot

Mark all files used in a standard build of bin-x86_64-efi/snponly.efi
as permitted for UEFI Secure Boot.  These files represent the core
functionality of iPXE that is guaranteed to have been included in
every binary that was previously subject to a security review and
signed by Microsoft.  It is therefore legitimate to assume that at
least these files have already been reviewed to the required standard
multiple times.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/iobuf.c

@@ -22,6 +22,7 @@
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( PERMITTED );
 
 #include <stdint.h>
 #include <strings.h>