[Arista] Better handle arbitrary tmpfs in boot0

[Staphylo]

- Why I did it

To limit IO and space usage on the flash device the boot0 script makes sure the SWI is in memory.
Because SONiC maps /tmp on the flash, some logic is required to make sure of it.
However it is possible for some provisioning mechanism to already download the swi in a memory file system.
This was not properly handled by the boot0 script.
It now properly detect if the image is on a tmpfs or a ramfs and keep it there if that is the case.

- How I did it

Check the filesystem on which the SWI pointed by swipath lies.
If this filesystem is a ramfs or a tmpfs the move_swi_to_tmpfs becomes a no-op.
Made sure the cleanup logic would not behave unexpectedly.

- How to verify it

In SONiC:

1. Download the swi under /tmp and makes sure it gets moved to /tmp/tmp-swi which gets mounted for that purpose.
   Make sure /tmp/tmp-swi gets unmounted once the install process is done.

2. Create a new mountpoint under /ram using either ramfs or tmpfs and download the swi there.
   Install the swi using sonic-installer and makes sure the image doesn't get moved by looking at the logs.

- Which release branch to backport (provide reason below if selected)

None, master will receive an independent fix soon.

- Description for the changelog

Better handle arbitrary tmpfs mounts in boot0

[Staphylo]

retest vsimage please

[jleveque]

Retest vsimage please