Documentation: Add basic flashing tutorial for Lenovo

* Add basic flashing tutorial
** Describe internal and external flashing
** Describe flash supply diode protection
** Gives general advices on flashing
** Describe how to use flashrom --ifd
*  Describe basic flashing on Lenovo T4xx devices
** Describe how to disassemble and access the flash IC on T4xx
** Describe flash layout on Sandy Bridge and Ivy Bridge series.

Change-Id: Ia833e27f4e7d89ee32be9bed21a0c021839facec
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/27852
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

Documentation/flash_tutorial/ext_power.md

@@ -0,0 +1,28 @@
+# Flashing firmware externally supplying direct power
+
+**WARNING:** Never use a high current rated power supply, like PC ATX power
+             supply. It'll literally melt your PCB traces on short circuit.
+
+On some mainboards the flash IC Vcc pin is connected to a diode, which prevents
+powering the rest of the board.
+
+![][flash_ic_diode]
+
+Please have a look at the mainboard specific documentation for details.
+
+On those boards it's safe to use a programmer and supply power externally.
+
+**WARNING:** Verify that you apply the correct voltage!
+
+## USB programmer
+USB programmers are usually current limited by the host USB hub. On USB 2.0
+ports the limit is 500mA, which is sufficient to power the flash. Those are
+the best choice as they are stateless and have a fast power on reset cycle.
+
+## Single board computers (like BeagleBone Black / RPi)
+Be careful when connecting a flash chip, especially when using a Pomona
+test-clip. A short circuit or overcurrent (250mA) causes a brown-out reset,
+resulting in a reboot of the running operating system (and possible loss of
+remote shell).
+
+[flash_ic_diode]: flash_ic_diode.svg