pixel3_mainline: device.mk: make fstab device-specific

fstab.common was updated to add virt a/b and slotselect; pixel3 is not
quite ready for it, so temporarily make it device specific and remove
slotselect for now.

TODO: revert this while adding virt a/b support.

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Change-Id: Iaaada81e2d2fbdaca2ee630eb61422adaedbbb1d
2 files changed