warp7: hab: Set environment variable indicating HAB enable

warp7: hab: Set environment variable indicating HAB enable

This patch adds an environment variable called "hab_enabled" which gets set
to a boolean status indicating whether HAB is enabled or not.

Subsequent patches can use this environment variable to determine if its
necessary to run a given binary through the hab_auth_img console command.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>