config_cmd_default.h: Add 'env exists' command

env exists allows scripts to query whether an environment variable
exists. Enable by default as it adds only a trivial amount of code and
can be useful in scripts.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>