minc-coat: Try to mount in-tree overlayfs if no overlay is found
minc-coat: Try to mount in-tree overlayfs if no overlay is found

Before loading kmodule of overlay, grep -wq overlay /proc/filesystems
always fails, but it actually shown after we tried to mount it.
So we try to mount in-tree overlay if we can't found any "overlay"
string in the /proc/filesystems.


mhiramat/mincs