install: split off the environment setup from launch_and_wait_handler
install: split off the environment setup from launch_and_wait_handler

This will allow us to reuse this code for other handlers.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc