Install: split build targets (#656)
Install: split build targets (#656)

Allows one to use `make -f Makefile.release build-arm` to just get an
Arm binary.
ii/coredns,yongtang/coredns,jremond/coredns,chrisohaver/coredns,johnbelamaric/coredns,jremond/coredns,johnbelamaric/coredns,miekg/coredns,miekg/coredns,varyoo/coredns,coredns/coredns,ii/coredns