targets/core: Set proxy variables based on previous proxy setting

On update, obtain *_proxy variables from the previously generated
environment, and export them after sanitizing the value (to make
sure we do not eval random harmful stuff).

Fixes #883.
Fix check for default variable values.
targets/core: Just source the new environment file from the installer

Simpler, should be safe enough.
Unit testing: add tool to import functions from scripts
Shuffle release order for each test.
xdg-utils is necessary for proper functioning of the croutonpowerd daemon
Merge pull request #911 from drinkcat/fix-proxy-env

targets/core: Set proxy variables based on previous proxy setting
Merge pull request #916 from dnschneid/randomrelorder

Shuffle release order for each test.
Merge pull request #915 from dnschneid/unittesting

Unit testing: add tool to import functions from scripts
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.
test/tests/19-verifykernel: Make use of import to run verifykernelsig

Also move the function back to installer/main.sh: we do not need
it in installer/functions anymore.