[sysupgrade] Don't try to pivot to new ramfs if already running on one

This patch allows the sysupgrade script to continue with the upgrade if
openwrt is already running on a ramfs. This allows the use of the sysupgrade
script as an _installer_ when running from an iso image. A user could boot the
system from a bootable cd and invoke sysupgrade -n <URL> to write an image to
the hard disk or CF.

Signed-off-by: Vasilis Tsiligiannis <b_tsiligiannis@silverton.gr>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@15683 3c298f89-4303-0410-b956-a3cf2f4a3e73