When submitting binaries for UEFI Secure Boot signing, certain
known-dubious subsystems (such as 802.11 and NFS) must be excluded
from the build.  Mark the directories containing these subsystems as
insecure, and allow the build target to include an explicit "security
flag" (a literal "-sb" appended to the build platform) to exclude
these source directories from the build process.

For example:

  make bin-x86_64-efi-sb/ipxe.efi

will build iPXE with all code from the 802.11 and NFS subsystems
excluded from the build.

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