[build] Exclude external files from annotation checks

[build] Exclude external files from annotation checks

External files such as embedded scripts or X.509 certificates are not
expected to include source file annotations such as FILE_LICENCE() or
FILE_SECBOOT().  Exclude these external files from the list of
annotated files used to perform licensing and UEFI Secure Boot
eligibility checks.

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