sysrepo: Implement parts of ietf-system module

Implement announcing OS name and OS release through ietf-system YANG
model [1], specifically via its top-level container system-state.
The properties are obtained from /etc/os-release file.

[1] https://tools.ietf.org/html/rfc7317

Change-Id: I82d5fd54659ea365232a3a9455dd73f84b8fd0d1
diff --git a/tests/system/missing-equal b/tests/system/missing-equal
new file mode 100644
index 0000000..ad16e1c
--- /dev/null
+++ b/tests/system/missing-equal
@@ -0,0 +1,2 @@
+VERSION
+NAME