Demo/E407: bringup on the Olimex STM32-E407 with all-GNU toolchain (WIP)

This is without involving FreeRTOS for now. Doesn't work yet.
Demo/E407/Makefile: add verbosity control, dependencies
Demo/E407: use M4F as M4F, not as M3

Turns out the FPU issues come into play later. So let's try to have a
clean setup that may also benefit others.
Demo/E407/Makefile (clean): also remove dependency files