Makefile improvements.
Makefile improvements.

- Binaries are not hardcoded anymore
- PREFIX is replaced by DESTDIR (standard convention)
- PREFIX-less targets, e.g. make bin/swarmd
- make install support

Signed-off-by: Andrea Luzzardi <442ffe3ffbfd024afcf0b7f8b108d05bca3aae43@gmail.com>

sungwonh/swarmkit,allencloud/swarmkit,dperny/swarmkit-1,allencloud/swarmkit,AliyunContainerService/swarmkit,Noyan759/swarmkit,fate-grand-order/swarmkit,Noyan759/swarmkit,yongtang/swarmkit,aaronlehmann/swarmkit,docker/swarmkit,deljuven/swarmkit,diogomonica/swarmkit,aaronlehmann/swarmkit,elifa/swarmkit,cezarsa/swarmkit,yp-engineering/swarmkit,elifa/swarmkit,RenaudWasTaken/swarmkit,AliyunContainerService/swarmkit,runshenzhu/swarmkit,yongtang/swarmkit,docker/swarmkit,cheyang/swarmkit,dperny/swarmkit-1,runshenzhu/swarmkit,deljuven/swarmkit,alyyousuf7/swarmkit,cezarsa/swarmkit,jlhawn/swarmkit,RenaudWasTaken/swarmkit,fate-grand-order/swarmkit,jlhawn/swarmkit,vasil-yordanov/swarmkit,diogomonica/swarmkit,yp-engineering/swarmkit,sungwonh/swarmkit,cheyang/swarmkit,alyyousuf7/swarmkit,vasil-yordanov/swarmkit