test/30-audio: Install recommended deps for pulseaudio

pulseaudio 5.0 on jessie (and probably sid) requires dbus-daemon to
be running in its default configuration. However,
`install --minimal pulseaudio` does not pull in dbus.

Removing --minimal ensures dbus is installed, as it would be in
most real cases.
Merge pull request #783 from drinkcat/jessie-audio

test/30-audio: Install recommended deps for pulseaudio
test/upgrade: Test multiple upgrade path in Ubuntu

Originally, we tested only precise->saucy, through non-LTS releases.

Now, we test quantal->saucy->trusty (this skips raring, as it was
EOL before quantal), raring->saucy, and LTS upgrade
precise->trusty.

precise->quantal is broken (upstream problem due to, at least,
libxi6 needing to be downgraded), but I don't see why anyone would
still want to do that now that trusty has been released.

Finally, the test only runs if `-R` is not specified on the
command line: we do not want to check against SUPPORTED_RELEASES
as this test intentionally tries out releases that are marked as
unsupported.
Merge pull request #787 from drinkcat/upgrade-test

test/upgrade: Test multiple upgrade path in Ubuntu
Revert "Temporary workaround for #777 while we investigate googlesource.com"

This reverts commit a003c45ebfe966383b2715e234e12e319cba140c.
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.