ARM: configs: cm_fx6: improve default environment

Currently, entire script segments have to be changed in the default
environment to change the kernel image location or to append kernel
cmdline parameters. In the later case this has to be changed for
every possible boot device.

Introduce new variables for kernel image locations and boot device
independent kernel parameters to make it easier to change these
settings.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Igor Grinberg <grinberg@compulab.co.il>
Reviewed-by: Nikita Kiryanov <nikita@compulab.co.il>