buildsteps: allow oe-init-build-env to be changed

Some distributions don't use oe-init-build-env but have their own environment
script.  Add a env_cmd property to RunPreamble to specify the command
(defaulting to oe-init-build-env) and if it works, set it as a property for
other steps to use.

Signed-off-by: Ross Burton <ross.burton@intel.com>
CreateAutoConf: really force the sstate, so the distro can't override it
buildset-config: add builds for the devkit
yocto-slave/buildbot.tac: set basedir to /data/yocto-autobuilder
CreateAutoConf: don't set TMPDIR now slaves are located in /data
autobuilder.conf: allow build publishing
twistd.hostname: delete, moved to /data
buildset-config: build a smaller devkit image for testing
autobuilder.conf: move all paths to /data
buildsteps: don't hard-code the slave base directory location

The slave base directory can be changed via yocto-slave/buildbot.tac, for
example to put the build directories on a separate partition to the checkout of
yocto-autobuilder.

To respect this, instead of constructing the slave directory from
$PWD/yocto-slave/$buildername use the build step "builddir" property which is
slave-base-directory/builder-name.

Signed-off-by: Ross Burton <ross.burton@intel.com>
buildset-config: disable GetDistroConfig for devkit and build images in world