[build] Fail Secure Boot builds unless all files are permitted

[build] Fail Secure Boot builds unless all files are permitted

Add the Secure Boot permissibility check as a dependency for targets
built with the Secure Boot flag enabled.  Attempting to build e.g.

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

will now fail unless all files used in the final binary are marked as
being permitted for Secure Boot.

This does not affect the standard build targets (without the "-sb"
suffix on the build directory).

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