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
Merge pull request #1754 from OlegHahm/timex_makefile_order

make: fix the order of Makefile.dep for timex