[aboot]: Better handle tmpfs management in boot0 (#6268)

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:

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.

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.

files/Aboot/boot0.j2

@@ -130,10 +130,27 @@ update_next_boot() {
     fi
 }
 
+is_in_memory() {
+    local file="$1"
+    local filedev=$(df "$file" | tail -n 1 | tr -s ' ' | cut -f6 -d' ')
+    local filedevfs=$(mount | grep " $filedev " | cut -f5 -d' ')
+
+    case "$filedevfs" in
+       tmpfs|ramfs) return 0;;
+       *) return 1;;
+    esac
+}
+
 move_swi_to_tmpfs() {
     local oldswi="$1"
     local newswi="$swi_tmpfs/$(basename $oldswi)"
 
+    # check if the swi is already in memory
+    if ! $in_aboot && is_in_memory "$oldswi"; then
+       echo "$oldswi"
+       return 0
+    fi
+
     mkdir -p "$swi_tmpfs"
     if ! $in_aboot && ! mount | grep -q ' /tmp type tmpfs'; then
         # mount a real tmpfs on /tmp/tmp-swi if /tmp is not one already.