Revert "CreateAutoConf.py: Support setting TMPDIR"

Setting TMPDIR like this doesn't work because too many build steps assume that
the build output is in slavebuilddir/build/build/.

This reverts commit 96b0b26bc52b2542f08831b6bcc1c109e431c2ba.

Signed-off-by: Ross Burton <ross.burton@intel.com>
CreateAutoConf: add inherits argument
CreateAutoConf: really force the sstate, so the distro can't override it
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>