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-core/systemd/obmc-targets/obmc-power-start@.target b/recipes-core/systemd/obmc-targets/obmc-power-start@.target
index a977d5a..720548e 100644
--- a/recipes-core/systemd/obmc-targets/obmc-power-start@.target
+++ b/recipes-core/systemd/obmc-targets/obmc-power-start@.target
@@ -1,7 +1,7 @@
 [Unit]
 Description=Power%i On
 After=obmc-power-start-pre@%i.target
-Wants={SYSTEMD_DEFAULT_TARGET}
+Wants=multi-user.target
 Conflicts=obmc-chassis-poweroff@%i.target
 RefuseManualStart=yes
 RefuseManualStop=yes