[stm32f429i-discovery] Moved Chucks usart irq example to usart irq console.

Chucks irq example is more elaborate than the ported one so we keep
both.

examples/stm32/f4/stm32f429i-discovery/usart_irq_console/README

@@ -0,0 +1,27 @@
+USART IRQ
+---------
+
+This is a slightly fancier console interface using interrupts. Basically
+this example sets up the USART for serial input and output as before
+except that the receive side is interrupt driven. That means you program
+won't miss a character if it happens to be taking its time printing something
+at the time.
+
+I've demonstrated this by setting it up so that if you type ^C to the
+program it causes an interrupt to occur that resets the program back
+to the start. This is done in a slightly tricky way to accomodate the
+Cortex M architecture. When you are interrupted in the Cortex M, the 
+proccessor goes into "Handler" mode, saves information on the stack,
+and takes the interrupt. Part of this involves saving a special value
+on the stack in the LR register. The useful thing is that it means you
+can write interrupt service routines as regular C code, the not so useful
+thing is that if you don't return from that stack, the processor gets
+confused about what state it should be in. So to avoid confusing the
+processor, the interrupt routine changes where the code will return, then
+does the return. This pops the processor out of Handler mode and back into
+Thread mode, at which point the C code can do a longjump.
+
+In the future, the C library may be able to note that the processor needs
+to change state for you, and save the special gymnastics here, but in the
+mean time this works and will continue to work in the future.
+