Allow bypassing default 10 s timeout in `sm-prepare-image -y`

[mmalecki]

There seems to be no publicly documented way of bypassing the default 10 s timeout in sm-prepare-image, even when passing -y.

(I think) this is possible to work around by passing private variables like: yn_start=y yn_nfs=y yn_subips=y yn_halt=n sm-prepare-image && poweroff.