Check return values of erase and tftp/cp

The _update script does not check the return value of erasing a partiton or
flashing the image to the device. Added this check to get a evaluable return
value from the update script.

Signed-off-by: Teresa Gámez <t.gamez@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

defaultenv/bin/_update

@@ -35,16 +35,16 @@ unprotect $part
 echo
 echo "erasing partition $part"
 echo
-erase $part
+erase $part || exit 1
 
 echo
 echo "flashing $image to $part"
 echo
 
 if [ x$mode = xtftp ]; then
-	tftp $image $part
+	tftp $image $part || exit 1
 else
-	cp $image $part
+	cp $image $part || exit 1
 fi
 
 protect $part