bitbake.conf: drop VOLATILE_TMP_DIR, use FILESYSTEM_PERMS_TABLES instead

bitbake.conf: drop VOLATILE_TMP_DIR, use FILESYSTEM_PERMS_TABLES instead

* Drop VOLATILE_TMP_DIR, use FILESYSTEM_PERMS_TABLES instead. By default,
FILESYSTEM_PERMS_TABLES ?=  "files/fs-perms.txt \
                             files/fs-perms-volatile-log.txt \
                             files/fs-perms-volatile-tmp.txt"
it contains 'files/fs-perms-volatile-tmp.txt', which means volatile tmp
is enabled. User can disable volatile tmp by remove
'files/fs-perms-volatile-tmp.txt' from FILESYSTEM_PERMS_TABLES.
* If volatile tmp is disabled, both /tmp and /var/tmp are persistent

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>