discover: Allow load_async_url() to call callback for local paths

discover: Allow load_async_url() to call callback for local paths

Several pxe-parser tests fail because the test harness's version of
load_async_url() will call the callback directly, but in pxe-parser the
caller checks if the path was local and calls the callback immediately.
Being called twice, a use-after-free occurs in the callback.

For consistency change the load_async_url() semantics such that it is
possible for load_async_url() to call the callback before it returns in
the case of local paths. Callers need to know this is possible, but now
won't need to check to call it manually.

This requires a slight reorganisation of the boot_process() code, since
it checks the result of several asynchronous load operations in the same
callback, and with this change not all of those results will necessarily
be initialised at callback time. Add a list of 'boot_resources' which
carry the required information for the resource and allow the boot
handler to treat different resources generically.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>