build: add comments to makefiles

Add comments to build system makefile functions and variables to help
developers in understanding build system internals and ease the
development process.

This patch adds some documentation examples with proposed doxygen-like
syntax. Hopefully, this would start the discussion and result in
generation of the makefile documentation guidelines.

Signed-off-by: Sergey Matsievskiy <matsievskiysv@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16888
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

include/target.mk

@@ -7,10 +7,17 @@ ifneq ($(__target_inc),1)
 __target_inc=1
 
 
-# default device type
+##@
+# @brief Default device type ( basic | nas | router ).
+##
 DEVICE_TYPE?=router
 
-# Default packages - the really basic set
+##@
+# @brief Default packages.
+#
+# The really basic set. Additional packages are added based on @DEVICE_TYPE and
+# @CONFIG_* values.
+##
 DEFAULT_PACKAGES:=\
 	base-files \
 	ca-bundle \
@@ -27,15 +34,21 @@ DEFAULT_PACKAGES:=\
 	urandom-seed \
 	urngd
 
-# For the basic set
+##@
+# @brief Default packages for @DEVICE_TYPE basic.
+##
 DEFAULT_PACKAGES.basic:=
-# For nas targets
+##@
+# @brief Default packages for @DEVICE_TYPE nas.
+##
 DEFAULT_PACKAGES.nas:=\
 	block-mount \
 	fdisk \
 	lsblk \
 	mdadm
-# For router targets
+##@
+# @brief Default packages for @DEVICE_TYPE router.
+##
 DEFAULT_PACKAGES.router:=\
 	dnsmasq \
 	firewall4 \
@@ -85,7 +98,18 @@ endif
 # Add device specific packages (here below to allow device type set from subtarget)
 DEFAULT_PACKAGES += $(DEFAULT_PACKAGES.$(DEVICE_TYPE))
 
+##@
+# @brief Filter out packages, prepended with `-`.
+#
+# @param 1: Package list.
+##
 filter_packages = $(filter-out -% $(patsubst -%,%,$(filter -%,$(1))),$(1))
+
+##@
+# @brief Append extra package dependencies.
+#
+# @param 1: Package list.
+##
 extra_packages = $(if $(filter wpad wpad-% nas,$(1)),iwinfo)
 
 define ProfileDefault