[POWERPC] bootwrapper: Bail from script if any command fails

Add the 'set -e' command to the wrapper script so that if any command
fails then the script will automatically exit

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>