Now that we have a bootm command which boots everything we can
simplify the defaultenvironment. We can call bootm on every
image type and can remove the kernelimage_type variables.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>