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/gpio/phosphor-gpio-monitor/phosphor-gpio-presence@.service b/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-presence@.service
index df75824..acb6af7 100644
--- a/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-presence@.service
+++ b/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-presence@.service
@@ -10,4 +10,4 @@
 SyslogIdentifier=phosphor-gpio-presence
 
 [Install]
-RequiredBy={SYSTEMD_DEFAULT_TARGET}
+RequiredBy=multi-user.target