Added support for reboot task

Test: tested on raven device
Change-Id: I4935d720f567e70da95ea8da37f3404b80b313c8
Bug: 194686221

Changed reboot {target} to work off tasks. reboot-{target} commands are
also supported.

Test: tested reboot on raven device
Change-Id: I05aed269d121a5d651c1ab1180a1b4878ae213fd

Modified load_buf to be able to find images in $OUT directory

Test: tested flash {partition} and flash {partition} {img_name} on raven
device

Change-Id: I3879792d11ad15bc910670853d2a7fe200fcc66f