discover/boot: Implement asynchronous boot() process

The current boot process handled by boot() function is implemented such
that it blocks on the boot resources being downloaded which blocks
the discover process until the download completes. This patch
re-implements the boot() function using callback mechanism and
asynchronous notifications so that boot() returns to continue the
discover process while the download is in progress.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>