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_standalone.md

@@ -0,0 +1,23 @@
+# Flashing firmware standalone
+
+If none of the other methods work, there are three possibilities:
+
+## Desolder
+You must remove or desolder the flash IC before you can flash it.
+It's recommended to solder a socket in place of the flash IC.
+
+When flashing the IC, always connect all input pins.
+If in doubt, pull /WP, /HOLD, /RESET and alike up towards Vcc.
+
+## SPI flash emulator
+If you are a developer, you might want to use an [EM100Pro] instead, which sets
+the onboard flash on hold, and allows to run custom firmware.
+It provides a very fast development cycle without actually writing to flash.
+
+## SPI flash overwrite
+It is possible to set the onboard flash on hold and use another flash chip.
+Connect all lines one-to-one, except /HOLD. Pull /HOLD of the soldered flash IC
+low, and /HOLD of your replacement flash IC high.
+
+
+[EM100Pro]: https://www.dediprog.com/product/EM100Pro