Use include script for common environment setting
Do not use "sudo" to install user-space components

User-space libraries destinations should be writeable by the user just
fine. Do not invoke sudo when installing them.
Fix compilation prefix

The prefix path must be relative to the root of the target filesystem.
Fix it, and rely on the DESTDIR variable defined in env.sh to place the
components in the right place (make install will use it as a destination
if it exists).
Build user-space components in out/
Add pthread-stubs

Pthread-stubs is required by drm and is not present in every toolchain.
download-rootfs: ensure destination directory exists

Create destination directory is needed, for the cases where user-space
is build without the kernel components.