[efi] Increase download timeout for autoexec.ipxe

[efi] Increase download timeout for autoexec.ipxe

In almost all cases, the download timeout for autoexec.ipxe is
irrelevant: the operation will either succeed or fail relatively
quickly (e.g. due to a nonexistent file).  The overall download
timeout exists only to ensure that an unattended or headless system
will not wait indefinitely in the case of a degenerate network
response (e.g. an HTTP server that returns an endless trickle of data
using chunked transfer encoding without ever reaching the end of the
file).

The current download timeout is too short if PeerDist content encoding
is enabled, since the overall download will abort before the first
peer discovery attempt has completed, and without allowing sufficient
time for an origin server range request.

The single timeout value is currently used for both the download
timeout and the sync timeout.  The latter timeout exists only to allow
network communication to be gracefully quiesced before removing the
temporary MNP network device, and may safely be shortened without
affecting functionality.

Fix by increasing the download timeout from two seconds to 30 seconds,
and defining a separate one-second timeout for the sync operation.

Reported-by: Michael Niehaus <niehaus@live.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>