Add thin abstraction layer for processes and filesystem

This is used instead of some hardcoded Unix calls and means this code
now works on Windows

(This is a rework of a previous patch which used boost, now that I have
been informed that we are trying to move towards removing boost).

Change-Id: Ib0d11055279bbd7b710f086e9890369e3ecbfe9a
Signed-off-by: Robert Hughes <robert.hughes@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e39c2b8..14c2c0c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -40,6 +40,10 @@
     include/armnnUtils/Permute.hpp
     include/armnnUtils/FloatingPointConverter.hpp
     include/armnnUtils/TensorUtils.hpp
+    src/armnnUtils/Filesystem.hpp
+    src/armnnUtils/Filesystem.cpp
+    src/armnnUtils/Processes.hpp
+    src/armnnUtils/Processes.cpp
     src/armnnUtils/GraphTopologicalSort.hpp
     src/armnnUtils/Half.hpp
     src/armnnUtils/Permute.cpp