Demo/Shared/Contiki/: add own Contiki top-level Makefile

This way, we no longer depend on contiki-outoftree's Makefile.
It also enables us to make our own additions.
Demo/Shared/Contiki/: new variable CONTIKI_EXTRA_CFLAGS_PREPEND

Like the other CONTIKI_EXTRA_* variables, CONTIKI_EXTRA_CFLAGS_PREPEND
is passed into the Contiki build, where its content is prepended to
CFLAGS. Its main use is to set up the beginning of the include path
(-I).