mainboard/lenovo/x230: Add ThinkPad x230s as a variant

The code is based on autoport and that for X230. Major differences are:
    - Only one DDR3 slot
    - HM77 PCH
    - M.2 socket instead of mini PCIe
    - No docking
    - No TPM

Tested:
    - CPU i5-3337U
    - 8GiB SO-DIMM
    - Camera
    - PCIe and USB2 on M.2 slot with A key for WLAN
    - SATA and USB2 (no SuperSpeed components) on M.2 slot with B key for WWAN
    - On board SDHCI connected to PCIe
    - USB3 ports
    - libgfxinit-based graphics init
    - NVRAM options for North and South bridges
    - Sound
    - ThinkPad EC
    - S3
    - Linux 4.9 within Debian GNU/Linux stable, loaded from SeaBIOS.

Untested:
    - Touch screen, which is said to work under ubuntu but not debian.

Change-Id: Id59cdc5479aaf70809dd1ca613056263661455eb
Signed-off-by: Bill XIE <persmule@hardenedlinux.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41390
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/mainboard/index.md

@@ -102,6 +102,7 @@ The boards in this section are not real mainboards, but emulators.
 - [W530](lenovo/w530.md)
 - [T430 / T530 / X230 / W530 common](lenovo/Ivy_Bridge_series.md)
 - [T431s](lenovo/t431s.md)
+- [X230s](lenovo/x230s.md)
 - [Internal flashing](lenovo/ivb_internal_flashing.md)
 
 ### Haswell series

Documentation/mainboard/lenovo/Ivy_Bridge_series.md

@@ -1,6 +1,6 @@
 # Lenovo Ivy Bridge series
 
-This information is valid for all supported models, except T430s and T431s.
+This information is valid for all supported models, except T430s, [T431s](t431s.md) and [X230s](x230s.md).
 
 ## Flashing coreboot
 ```eval_rst

Documentation/mainboard/lenovo/x230s.md

@@ -0,0 +1,19 @@
+# ThinkPad Lenovo X230s
+
+## Disassembly Instructions
+
+You must remove the following parts to access the SPI flash chip:
+
+![x230s_bc_removed](x230s_bc_removed.jpg)
+
+* Base cover
+
+The [Hardware Maintenance Manual](https://download.lenovo.com/ibmdl/pub/pc/pccbbs/
+mobiles_pdf/x230s_hmm_en_0c10860_01.pdf) could be used as a guidance of disassembly.
+
+The SPI flash chip (W25Q128.V in the form of SOIC-8 for the author's X230s, but varying is possible)
+is located at the circled place.
+
+Unlike [most Ivy Bridge ThinkPads](Ivy_Bridge_series.md), X230s has a single 16MiB SPI flash chip.
+
+The general [flashing tutorial](../../flash_tutorial/index.md) has more details.