Demo/E407/Makefile: move STM32F4xx references to variables; add STM32F2xx example

We should now be able to use this as a general-purpose Makefile for
STM32F2xx, STM32F4xx, and perhaps even STM32F1xx
E407/: move STM32-generic items to Makefile.stm32, generalize CFLAGS, cleanup
E407/Makefile.stm32 (spotless): only suggest removal of stm32f2-f4_demobuild.zip

... but leave the killing to the user.