doc: add documentation on default configurations

doc/doxygen/src/getting-started.md

@@ -153,6 +153,18 @@ For instructions on how to configure via `CFLAGS` check the
 @ref config "identified compile-time configurations". To learn how to use
 Kconfig in RIOT, please refer to the @ref kconfig-users-guide.
 
+Default configurations                                 {#default-configurations}
+----------------------
+When devices have a common access interface, having a default configuration to
+enable them across platforms, without having to explicitly specify which modules
+to include, comes in handy. For this, two pseudomodules are defined:
+
+- `saul_default`: will enable all the drivers of sensors and actuators that are
+present in the target platform.
+
+- `netdev_default`: will enable all the drivers of network devices
+present in the target platform.
+
 Use Docker to build RIOT                           {#docker}
 ========================
 [Docker](https://www.docker.com/) is a platform that allows packaging software into containers that can easily be run on any Linux that has Docker installed.

doc/doxygen/src/porting-boards.md

@@ -105,6 +105,29 @@ endif
 the dependency block for your board *before* its dependencies pull in their own
 dependencies.
 
+#### Default configurations
+As explained in @ref default-configurations "Default Configurations", there are
+two pseudomodules that are used to indicate that certain drivers of devices
+present in the platform should be enabled. Each board (or CPU) has knowledge as
+to which drivers should be enabled in each case.
+
+The previous code snippet shows how a board which has a @ref drivers_sx127x
+device, pulls in its driver when the default network interfaces are required.
+
+When the pseudomodule `saul_default` is enabled, the board should pull in all
+the drivers of the devices it has which provide a @ref drivers_saul interface. This is
+usually done as following:
+
+```mk
+ifneq (,$(filter saul_default,$(USEMODULE)))
+  USEMODULE += saul_gpio
+  USEMODULE += apds9960
+  USEMODULE += bmp280_i2c
+  USEMODULE += lis3mdl
+  USEMODULE += sht3x
+endif
+```
+
 ### Makefile.features                                       {#makefile-features}
 
 This file defines all the features provided by the BOARD. These features