Implement configurable system clock generation

Implement configurable system clock generation

PLL-HSE working

PLL-HSI working

Move SystemCoreClockUpdate in SystemInit to end

Switching from HSI-PLL to HSE-PLL (and back) is working

It works during various levels of overclocking.

Renamed CLI variable hse_mhz to system_hse_mhz

Restored the original position of the spectrum bind code

The internal logic of the spectrum bind code will prevent binding
process to fire if executed after soft reset.

Remove stale call to delay

Add a comment about call to spektrumBind placement

Declared SystemXXXSource functions, handled sign-ness warning.

Cleaned up commented out sections

USB clock generation for F446

Default HSE value for backward compatibility

Cleaned up more unused stuff

Handle non-F4 targets

Added comment about PLL_M selection

Removed fake gyro/acc from test target