Rename Makefile to Makefile.root

Some systems come with an old version of Make that is not compatible
with the projects built in this repo. To work around this issue, we will
add a proxy Makefile that will forward all calls to the real root
Makefile. As a first step, this CL moves Makefile to Makefile.root.

Warning: By itself this CL will break the build because temporarily
there is no root Makefile. The renaming was split off from the
subsequent CL to preserve the history of the file.

Test: n/a (does not build)
Change-Id: Id70c84a41318792a61b98fd23ea75111c2ba8e15
diff --git a/Makefile b/Makefile.root
similarity index 100%
rename from Makefile
rename to Makefile.root