make: Allow override of root path

Allows inclusion of Makefile.common from out of tree.

Signed-off-by: Michel Pollet <buserror@gmail.com>
test: Fix coroutine on old gcc

Removed traling colon. Fix issue #34

Signed-off-by: Michel Pollet <buserror@gmail.com>
cores: Fix mega2560

Added EIND, as per nnayo's. Also renamed the file to match the other
cores

Signed-off-by: Michel Pollet <buserror@gmail.com>