image_signing: fix key insert logic
image_signing: fix key insert logic

We don't want to override the common trap as the common sh files already
have handlers installed to clean up files/mounts.  Re-use those helpers
to avoid leaking loopback mounts.

BRANCH=None
BUG=chromium:718184
TEST=signing images still works

Change-Id: I749ce5075194356219fea51152154fdc5a2e3b99
Reviewed-on: https://chromium-review.googlesource.com/505575
Reviewed-by: Eric Caruso <b4dfd424537681137c06263971140211e84cb3c6@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot