Remove tmpfs size and properly format generated fstab for arm

Remove tmpfs size limitation, this breaks make installworld and installation of some packages
Format generated fstab using tabs to make it consistent and readable

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29283

(cherry picked from commit 5bffdafd6c5f2a8279a57172ab760ea66ed3d7d5)