component/chromium: use system libudev and add a long comment for the explanation.

component/chromium/buildout.cfg

 [buildout]
+# chromium binary is linking to libudev, that can be provided by
+# component/systemd.  but current component/systemd can be built only
+# with Linux kernel >= 3.7. so libudev is not included in
+# LD_LIBRARY_PATH below intentionally and libudev should be installed
+# in the system.
+
 extends =
   ../coreutils/buildout.cfg
   ../cups/buildout.cfg
@@ -17,7 +23,6 @@ extends =
   ../nss/buildout.cfg
   ../pcre/buildout.cfg
   ../sqlite3/buildout.cfg
-  ../systemd/buildout.cfg
   ../xorg/buildout.cfg
   ../zlib/buildout.cfg
 
@@ -86,7 +91,6 @@ script =
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${pcre:location}/lib"
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${pixman:location}/lib"
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${sqlite3:location}/lib"
-  export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${systemd:location}/lib"
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${xdamage:location}/lib"
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${xfixes:location}/lib"
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${zlib:location}/lib"