populate-volatile.sh: use $ROOT_DIR/var/volatile/tmp as TMPDIR

populate-volatile.sh: use $ROOT_DIR/var/volatile/tmp as TMPDIR

It's possible that a 'No such directory' error occurs when doing
check_requirement in populate-volatile.sh at rootfs time. This is
because the $ROOT_DIR/var/tmp might be a dead link.

Use $ROOT_DIR/var/volatile/tmp as the TMPDIR instead to avoid this
error.

[YOCTO #4883]
[YOCTO #4103]

(From OE-Core rev: 7c2c36a97bab46c73a3a1fb743ad3ec67a4c072e)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>