jem: Use /bin/echo to create the stack file

echo creates a stack file with the following incorrect contents:
-ne \x00\x50\x7c\x80
-ne \x00\x50\x7c\x80
..

This in turn caused a reboot to recovery mess up the stack/device
and the stock images needed to be restored via fastboot.

The use of /bin/echo fixes the problem.