add loopdev support.
ramfs,mtdfs is char device.
loopdev is a block device.
losetup make chardev to be visited as a block device.
you can set env loop to let loopdevopen auto run losetup,such as on bootup.
loopdev now support unzip.
git-svn-id: file:///svn/pmon-all/pmon-all@364 214b0138-1524-0410-9122-e5cb4b5bc56c