[build] Mark existing files as explicitly forbidden for Secure Boot

[build] Mark existing files as explicitly forbidden for Secure Boot

The third-party 802.11 stack and NFS protocol code are known to
include multiple potential vulnerabilities and are explicitly
forbidden from being included in Secure Boot signed builds.  This is
currently handled at the per-directory level by defining a list of
source directories (SRCDIRS_INSEC) that are to be excluded from Secure
Boot builds.

Annotate all files in these directories with FILE_SECBOOT() to convey
this information to the new per-file Secure Boot permissibility check,
and remove the old separation between SRCDIRS and SRCDIRS_INSEC.

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