[yocto] Specifying MACHINE when setting up build environment

[colin.helliwell at ln-systems.com]

Is there a way to specify what MACHINE is put into local.conf when first
setting up the build directory with oe-init-build-env?
I have my own TEMPLATECONF which I use, but I'd like to be able to set the
MACHINE on the command line rather than having to then modify local.conf