[blob] Add an abstraction of an openable data blob

Within the iPXE data transfer interface model, openers are fully
asynchronous and may not deliver any data until after the opener has
returned.

Provide a trivial openable data blob object (as a generalisation of
the "hello world" data transfer interface example code) that will
simply deliver a single fixed blob of data to its parent interface and
then close itself.

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