Split the core-OS and instance-specific env

Since we'll need to be able to generate the OS-specific environment
separately from the instance one, we move it to a separate function. We
also add a new OS_NAME env. var which is identical to the INSTANCE_OS
one (which won't exist for OS-only environments).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>