raspberrypi, raspberrypi2: document the two consoles (UART+HDMI)

The defconfigs for the Raspberry Pi boards start a console on HDMI in
addition to the classic one on UART. Document this feature in the
readme.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/raspberrypi/readme.txt

@@ -74,4 +74,6 @@ Copy the bootable "sdcard.img" onto an SD card with "dd":
   $ sudo dd if=output/images/sdcard.img of=/dev/sdX
 
 Insert the SDcard into your Raspberry Pi, and power it up. Your new system
-should come up now.
+should come up now and start two consoles: one on the serial port on
+the P1 header, one on the HDMI output where you can login using a USB
+keyboard.