scons: Minor cleanup of how partial linking is handled in makeEnv.

scons: Minor cleanup of how partial linking is handled in makeEnv.

Git rid of the unused partial_objs variable, and change append to
extend since the result of builders is a list of nodes and not
individual nodes, even if there's only one target.

Change-Id: Ie51225d3ea699eb7da18199ac3bf3e868e3994a6
Reviewed-on: https://gem5-review.googlesource.com/5985
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>