E407/Makefile, WM09/Makefile: move also DRV_DIR; rearrange some comments
WM09/demo.c (serial_receive): echo received characters as "x"

... to check that the interrupt handling works. It does :-)
E407/Makefile: move Contiki-specific items to Makefile.contiki
E407/Makefile.stm32: combine settings that contain the lower-case chip family
E407/Makefile, WM09/Makefile: move also DRIVERS_C to Makefile.stm32
WM09/demo.c: experimental GPIO-toggling code