Code review comment for lp://qastaging/~liuyq0307/lava-master-image-scripts/fix-trap-on-exit

In my environment, both bash and dash have the same error.
I guess there are some changes in the implementation of trap.
But trap is actually a built-in of bash or dash.
I don't know what is the real reason, but with the "-" argument it works for me.

14:08:46 liuyq:lava-master-image-scripts$ ./lava-create-master panda
LAVA master images are easy to make, just three steps:
Note: run with --verbose to see details

1) Preparing vanilla Linaro image
2) Converting vanilla image to LAVA master image
add map loop0p1 (253:0): 0 106496 linear /dev/loop0 8192
add map loop0p2 (253:1): 0 1982464 linear /dev/loop0 114688
trap: usage: trap [-lp] [[arg] signal_spec ...]
./lava-create-master: line 1: 18394 Terminated udisks --inhibit > /dev/null 2>&1
umount: /tmp/tmp.pLLTJFi1bQ.lava.panda.master: not found

14:09:04 liuyq:lava-master-image-scripts$ dash ./lava-create-master panda
LAVA master images are easy to make, just three steps:
Note: run with --verbose to see details

1) Preparing vanilla Linaro image
2) Converting vanilla image to LAVA master image
add map loop0p1 (253:0): 0 106496 linear /dev/loop0 8192
add map loop0p2 (253:1): 0 1982464 linear /dev/loop0 114688
trap: usage: trap [-lp] [[arg] signal_spec ...]
./lava-create-master: line 1: 18526 Terminated udisks --inhibit > /dev/null 2>&1
umount: /tmp/tmp.HfCnoniGT9.lava.panda.master: not found

14:09:23 liuyq:lava-master-image-scripts$ bash ./lava-create-master panda
LAVA master images are easy to make, just three steps:
Note: run with --verbose to see details

1) Preparing vanilla Linaro image
2) Converting vanilla image to LAVA master image
add map loop0p1 (253:0): 0 106496 linear /dev/loop0 8192
add map loop0p2 (253:1): 0 1982464 linear /dev/loop0 114688
trap: usage: trap [-lp] [[arg] signal_spec ...]
./lava-create-master: line 1: 18653 Terminated udisks --inhibit > /dev/null 2>&1
umount: /tmp/tmp.IVqlTEWDJK.lava.panda.master: not found
14:09:38 liuyq:lava-master-image-scripts$