[interface] Allow for the definition of an unused interface operation

[interface] Allow for the definition of an unused interface operation

Allow an interface operation to be declared as unused.  This will
perform full type-checking and compilation of the implementing method,
without including any code in the resulting object (other than a NULL
entry in the interface operations table).

The intention is to provide a relatively clean way for interface
operation methods to be omitted in builds for which the operation is
not required (such as an operation to describe an object using an EFI
device path, which would not be required in a non-EFI build).

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