Added default value for systemd_target meson option.

Added default value as multi-user.target for systemd_target
meson option in phosphor-host-postd. So, need not to pass
value from meta-phosphor for this meson option.

TESTED : Built Facebook YosemiteV2 images and loaded on
the target hardware. Verified all four host postcodes are
displayed.

Signed-off-by: Kumar Thangavel <thangavel.k@hcl.com>
Change-Id: I4878594b0b4876e99405a51af5fac3b2082b53ae
diff --git a/meson_options.txt b/meson_options.txt
index f1c0cf1..fb4460e 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -23,7 +23,8 @@
 option(
     'systemd-target',
     description: 'Target for starting this service.',
-    type: 'string'
+    type: 'string',
+    value: 'multi-user.target',
 )
 option(
     '7seg', type: 'feature', description: 'Enable building 7seg POST daemon.',