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>
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>
autobuilder.conf: local tweaks
CreateAutoConf: move tmpdir to /data
set hostname
master.cfg: set username/password
ross-x86.conf: add basic buildset
autobuilder: disable build history collection
ross-x86: fix layer, build more images, run at 5am
master.cfg: allow only one build at a time
buildset-config: add qemuarm build
buildset-config: build weston for x86
buildset-config: split arm to separate conf
ross-x86: don't build core-image-sato, just -dev
ross-nuc: add NUC target
yoctoAB: don't force an order
ross-x86: do a systemd build too
buildset-config: split systemd out of x86 to avoid work directory contamination
gitignore: ignore my build trees
buildset-config: fix layers in ross-nuc so it actually works