Add DL_DIR and SSTATE_DIR to passthrough environment

[ericblade]

• Allow users to specify DL_DIR and SSTATE_DIR from their environment
• Makes it easier to share downloads and state across multiple bitbake setups

[CLAassistant]

All committers have signed the CLA.

[shr-project]

Why don't you set them in local.conf or webos-local.conf?

[ericblade]

I find it to be much more convenient to be able to set them from environment, so then either myself or a build system doesn't have to remember or be instructed to edit conf files for each new checkout.

[shr-project]

You can also use webos-global.conf.