Demo/Shared/STM32/pll/: PLL calculator for STM32F2 and STM32F4 MCUs
Demo/Shared/STM32/Makefile.stm32: add more section comments; improve spacing
STM32/pll/pll.c: use getopt; fix USB 48 MHz check; move family selection to separate function

We use getopt so that the -u option can be placed after the family name,
which we'll need in Makefile.stm32
STM32/pll/Makefile: add "spotless" target; remove executable only on spotless, no clean
STM32/pll/pll.c: describe what the output means
unify system*.c generation under Shared/STM32 (uses PLL configurator)