installer: Make sure there is at least 1 target to be installed.

Ancient chroots did not provide a target list, this serves as a reminder.

Fixes #749 and #788.
Improve ze text.
test/16-targetsfile: Test if installer detects an absent or empty /etc/crouton/targets
Merge pull request #790 from drinkcat/update-error-no-target

installer: Make sure there is at least 1 target to be installed.
installer/functions: verifykernelsig, verify kernels signature.

Use dev_debug_vboot to figure out if dev_boot_signed_only should
be re-enabled.
test/tests/19-verifykernel: Test if kernel signature check is working.

The test should succeed on all autotest platforms, as they run
official images.
installer/main.sh: Check kernel verification before setting dev_boot_signed_only=1.

Since the test can take up to ~15 seconds to run, we run it in background,
and only update the flag after the bootstrap script has run, but before
the we enter the chroot for the first time.

We also move USB/legacy boot warnings to the end of the script.