boards/chromebooks: add sarien board documentation

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>

content/boards/chromebooks/boards/sarien.md

+---
+title: Sarien Chromebooks
+---
+
+`sarien` is a board name for x86_64-based Chromebooks. Some examples are:
+
+  - [Dell Latitude 5400 Chromebook Enterprise](https://www.dell.com/en-us/work/shop/dell-laptops-and-notebooks/dell-latitude-5400-chromebook-enterprise/spd/latitude-14-5400-chrome-laptop/xctolc540014us1)
+  - [Dell Latitude 5300 2-in-1 Chromebook Enterprise](https://www.dell.com/en-us/work/shop/cty/pdp/spd/latitude-13-5300-2-in-1-chrome-laptop)
+
+The specs for these chromebooks vary in terms of format, display, connectivity
+and devices, but they are based on Intel Whiskey Lake 64 bit CPUs
+such as the Celeron 4305U.
+
+### Debugging interfaces
+
+The Dell Latitude 5400 Chromebook Enterprise has been flashed and tested
+with a [Servo Micro](../../01-debugging_interfaces) interface and a
+RTL8152-based USB-Ethernet adapter. Note that these devices don't
+support [CCD](../../02-ccd).
+
+#### Network connectivity
+
+An external USB-Ethernet dongle based on the Realtek RTL8152 can be used
+to provide Ethernet connectivity in Depthcharge. Alternatively, A Servo v4 can be
+used together with the Servo Micro to provide an Ethernet interface,
+although this hasn't been tested yet.
+
+#### Known issues
+
+See [Common issues](../common_issues.md).
+
+### Example kernel command line arguments
+
+```
+earlyprintk=uart8250,mmio32,0xde000000,115200n8 console_msg_format=syslog console=ttyS0,115200n8 root=/dev/nfs ip=dhcp rootwait rw nfsroot=192.168.2.100:/srv/nfs/chromebook,v3 nfsrootdebug
+```
+
+The IP and path of the NFS share are examples and should be adapted to
+the test setup.