Setup a temporary directory for update_engine_sideload.

When running update_engine_sideload from recovery, the default temp
directory (/data/misc/update_engine/tmp) is not available. This
directory is currently used to store a blob when applying a bspatch
from a child process. This patch uses /tmp/update_engine_sideload as
a temporary directory when running update_engine_sideload from recovery.

Bug: 27178350
TEST=`adb sideload` an incremental update.

Change-Id: Iec9f4ec8314e8dae4f6b1e78402dd39ca49c4fa4