script: add overlayfs setup script for chaindata

[minh-bq]

overlayfs can used to quickly rollback all the changes we make to the chaindata when testing.

Example usage:

> pwd
/home/test

> ls
chaindata

> overlayfs_chaindata.sh -d /home/test
+ cd /home/test/
+ mkdir -p upper
+ mkdir -p workdir
+ mv chaindata orig_chaindata
+ mkdir -p chaindata
+ mount -t overlay overlay -olowerdir=./orig_chaindata,upperdir=./upper,workdir=./workdir chaindata

\# make changes to chaindata
...

\# rollback to original chaindata
> overlayfs_chaindata.sh -d /home/test -c
+ cd /home/minh_bui_skymavis_com/
+ umount ./chaindata
+ rm -rf chaindata
+ rm -rf upper
+ rm -rf workdir
+ mv orig_chaindata chaindata