tkn154: fixing #312 by using TOSMAKE_FIRST_FLAGS make variable

tkn154-mac relies on overlaying some of the standard platform modules.
The old tkn154 makefile system didn't work after the Tinyos makefile system
was redesigned. The introduction of TOSMAKE_FIRST_FLAGS now allows to
hand over makefile includes to the compiler before anything else.
Merge pull request #313 from mstaflex/master

tkn154: fixing #312 by using TOSMAKE_FIRST_FLAGS make variable