Ensure /etc/timezone also includes the TIMEZONE setting

Quoting from the bug report:

| The timezone is currently setup in chroot-script:timezone(), but
| that function only sets up /etc/localtime but not /etc/timezone.
| The problem is that you end up with one set to Europe/Vienna and
| the other set to Etc/UTC.
|
| This can greatly confuse the system, due to some libraries/tools
| parsing the former, and other the latter.
|
| So that function should probably also update /etc/timezone, with
| the TIMEZONE value.

So write the $TIMEZONE setting to /etc/timezone.

Closes: #904607
Thanks: Guillem Jover <gjover@sipwise.com>