make: fix the order of Makefile.dep for timex

The order of modules in Makefile.dep matters.
make: fix dependencies

uart0 depends on POSIX, not the other way around.
posix: configure stdio only to uart0 if enabled
boards/iot-lab_M3: fix PORT
Merge pull request #1754 from OlegHahm/timex_makefile_order

make: fix the order of Makefile.dep for timex
Merge pull request #1773 from thomaseichinger/fix_iot-lab_term

boards/iot-lab_M3: fix PORT for TERM
netdev: Provide maximum value of netdev_opt_t for further definitions
Merge pull request #1774 from authmillenon/netdev-opt-max

netdev: Provide maximum value of netdev_opt_t for further definitions