meson: create structure for use as submodule

Some other openbmc projects have set up sdbusplus to use as a
meson-subproject.  When porting one of them (entity-manager) to use
the meson build, I realized we are not structuring our meson.build
file according to best-practices for consumption as a submodule.
Specifically, we are missing the 'sdbusplus_dep' variable as described
in the meson reference: https://mesonbuild.com/Subprojects.html

Now that 'sdbusplus_dep' is defined, use it throughout the rest of
the meson.build files as well for simplification.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I79ad586aa185f769c7ecc83ef06e1f3897e8db2d
diff --git a/meson.build b/meson.build
index 41d9984..8ea6b9b 100644
--- a/meson.build
+++ b/meson.build
@@ -24,6 +24,12 @@
     install: true,
 )
 
+sdbusplus_dep = declare_dependency(
+    include_directories: root_inc,
+    link_with: libsdbusplus,
+    dependencies: libsystemd_pkg
+)
+
 subdir('tools')
 
 boost_dep = dependency(