Use search method instead of fixed offsets for finding rootfs. This unbreaks
recent images for picostation by allowing to find rootfs at correct location.

Suggested by: loos
MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273876 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f