docs/manual: describe relying on default options

People can assume that e.g. Busybox options enabled in the package are
enabled when writing code for Buildroot. Anyone who uses custom
configurations that disable default options needs to make sure
relevant scripts etc. still work for themselves.

Signed-off-by: Fiona Klute (WIWA) <fiona.klute@gmx.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/integration-configurable-packages.adoc

@@ -0,0 +1,20 @@
+// -*- mode:doc; -*-
+// vim: set syntax=asciidoc:
+
+[[configurable-packages]]
+=== Configurable packages
+
+Some foundational packages like Busybox and uClibc can be configured
+with or without certain features. When writing Buildroot code that
+uses such packages, contributors may assume that the options enabled
+in the Buildroot-provided configurations are enabled. For example,
++package/busybox/busybox.config+ sets
++CONFIG_FEATURE_START_STOP_DAEMON_LONG_OPTIONS=y+, so init scripts
+meant for use with Busybox init may use +start-stop-daemon+ with long
+form options.
+
+People who use custom configurations that disable such default options
+are responsible for making sure that any relevant scripts/packages
+still work, and if not, adapting them accordingly. To follow the
+Busybox example above, disabling long form options will require
+replacing init scripts that use them (in an overlay).

docs/manual/integration.adoc

@@ -9,6 +9,8 @@ level. Buildroot is highly configurable, almost everything discussed
 here can be changed or overridden by xref:rootfs-custom[rootfs overlay
 or custom skeleton] configuration.
 
+include::integration-configurable-packages.adoc[]
+
 include::integration-systemd.adoc[]
 
 include::integration-selinux-support.adoc[]