Fix the bug that tmpfs: Bad value for 'size'

[Unit]
Description=Create /tmp

[Mount]
What=tmpfs
Where=/tmp
Type=tmpfs
Options=nodev,nosuid,size=${TMP_SIZE},mode=${TMP_MODE}

[Install]
WantedBy=local-fs.target

the image was built based on isar-cip-core, but there is an error that tmpfs: Bad value for 'size'. And Maybe default variables are missing in recipes-core/tmp-fs/files/tmp.mount.tmpl in Options, bitbake does not expand them.

After felix investigated, this issue maybe link to ce100b3d.