[build] Exclude local named config headers from annotation checks

[build] Exclude local named config headers from annotation checks

Commit dee71adda ("[build] Exclude external files from annotation
checks") excluded local top-level config headers from annotation
checks, but not local named config headers.

These are generated if missing when building with CONFIG= and will
most of the time be empty.  Exclude these files from the list of
annotated files used to perform licensing and UEFI Secure Boot
eligibility checks.

Non-local named config headers intended to be used with Secure Boot
can be annotated with FILE_SECBOOT().

Signed-off-by: Christian I. Nilsson <ChristianN@2PintSoftware.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>