fs_config: replace getenv('OUT') by new fs_config parameter

Using a getenv('OUT') in such a deep down function is a wrong design
choice. Replacing with explicit parameter that may be NULL in case
device specific files can be accessed from /.
Since TARGET_COPY_OUT_SYSTEM may be defined to something different than
system we also ensure that we use a path relative to TARGET_OUT to
compute path to fs_config_* files.

Bug: 21989305
Bug: 22048934
Change-Id: Id91bc183b29beac7379d1117ad83bd3346e6897b
Signed-off-by: Thierry Strudel <tstrudel@google.com>