Add proxy root Makefile

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, a
previous CL moved the root Makefile to Makefile.root, and this CL adds
a proxy Makefile that forwards all calls to Makefile.root run with
a prebuilt of GNU Make.

Test: make test
Change-Id: I1ac0bfd2ab13971dfb1141e7c49491da47c234f3
1 file changed