meson: enable full subproject build

Misc changes to the build process and adding wrap-files so that
the full repository can be built as a meson subproject.  This
depends on a few pending changes upstream in phosphor-host-ipmid,
but since that subproject references HEAD it will automatically
resolve as those changes are merged.

Change-Id: Ia3d50e0efa21b07ae4d4886f5930a270976670ed
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/subprojects/sdbusplus.wrap b/subprojects/sdbusplus.wrap
new file mode 100644
index 0000000..42cfcee
--- /dev/null
+++ b/subprojects/sdbusplus.wrap
@@ -0,0 +1,6 @@
+[wrap-git]
+url = https://github.com/openbmc/sdbusplus
+revision = HEAD
+
+[provide]
+sdbusplus = sdbusplus_dep