mambo: Allow setting the Linux command line from the environment
mambo: Allow setting the Linux command line from the environment

For automated testing it's helpful to be able to set the Linux command
line via an environment variable.

Signed-off-by: Michael Ellerman <864f124608374e06e4da1fa5d2e47ed839b95411@ellerman.id.au>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/skiboot,qemu/skiboot,stewart-ibm/skiboot,mikey/skiboot,mikey/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,stewart-ibm/skiboot,legoater/skiboot,shenki/skiboot,stewart-ibm/skiboot,shenki/skiboot