meta-phosphor: move all services to multi-user.target

Instead of obmc-standby.target, use multi-user.target.  This can be
accomplished by changing the definition of SYSTEMD_DEFAULT_TARGET first,
however, the explosive approach by removing that variable and
hard-coding multi-user.target provides a single-step.

Note: This patchset represents effectively the wholesale change of all
services in the meta-phosphor layer as well as the targets.  There are
other parts of OpenBMC that leverage the default variable.

Change-Id: I2b3b2274dd6b32d3832d6e768c26b2607f0ce71c
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/recipes-phosphor/network/inarp/inarp.service b/recipes-phosphor/network/inarp/inarp.service
index 5d26559..6e85131 100644
--- a/recipes-phosphor/network/inarp/inarp.service
+++ b/recipes-phosphor/network/inarp/inarp.service
@@ -7,4 +7,4 @@
 Restart=always
 
 [Install]
-WantedBy={SYSTEMD_DEFAULT_TARGET}
+WantedBy=multi-user.target