Create first draft for buildimage readme

README.buildsystem.md

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+# SONiC Buildimage Guide
+## Overview
+SONiC build system is a *GNU make* based environment for build process automation.
+It consists of two main parts:
+ * Backend - collection of makefiles and other helpers that define generic target groups, used by recipes
+ * Frontend - collection of recipes, that define metadata for each build target
+
+## Structure
+File structure of SONiC Buildimage is as follows:  
+```
+sonic-buildimage/  
+  Makefile  
+  slave.mk  
+  sonic-slave/  
+    Dockerfile  
+  rules/  
+    config  
+    functions
+    recipe1.mk
+    ..
+  dockers/  
+    docker1/  
+      Dockerfile.template    
+    ..  
+  src/  
+    submodule1/    
+    ..  
+    package1/  
+      Makefile  
+    ..  
+  platform/  
+    vendor1/  
+    ..  
+  target/  
+    debs/  
+    python-wheels/  
+```
+### Backend  
+**Makefile**, **slave.mk** and **sonic-slave/Dockerfile** are the backend of buildimage.  
+*slave.mk* is the actual makefile. It defines a set of rules for *target groups* (more on that later).
+You can find a make rules for every target that is defined in recipe there.  
+*Makefile* is a wrapper over sonic-slave docker image.
+Every part of build is executed in a docker container called sonic-slave, specifically crafted for this environment.
+If build is started for the first time, a new sonic-slave image will be built form *sonic-slave/Dockerfile* on the machine.
+It might take some time, so be patient.
+After that all subsequent make commands will be executed inside this container.
+*Makefile* takes every target that is passed to make command and delegates it as an entry point to a container,
+making process of running container transparent.  
+  
+### Frontend  
+**rules/** has a collection of recipes for platform independent targets.
+Every recipe is a file that describes a metadata of a specific target, that is needed for its build.  
+You might find **rules/config** very useful, as it is a configuration file for a build system, which enables/disables some tweaks.  
+**dockers/** directory is a place where you can find Dockerfiles for generic docker images.  
+**src/** is a place where a source code for generic packages goes.
+It has both submodules (simple case, just run dpkg-buildpackage to build),
+and directories with more complcated components, that provide their own Makefiles.  
+**platform/** contains all vendor-specific recipes, submodules etc.  
+Every **platform/[VENDOR]/** directory is a derived part of buildimage frontend, that defines rules and targets for a concrete vendor.  
+
+### Build output
+**target/** is basically a build output. You can find all biuld artifacts there.  
+## Recipes and target groups
+Now let's go over a definition of recipes and target groups.  
+*Recipe* is a small makefile that defines a target and set of variables for building it.  
+*Target group* is a set of targets that are built according to the same rulels.  
+
+### Recipe example
+Lets take a recipe for swss as an example:  
+```make
+# libswsscommon package
+
+LIBSWSSCOMMON = libswsscommon_1.0.0_amd64.deb
+$(LIBSWSSCOMMON)_SRC_PATH = $(SRC_PATH)/sonic-swss-common
+$(LIBSWSSCOMMON)_DEPENDS += $(LIBHIREDIS_DEV) $(LIBNL3_DEV) $(LIBNL_GENL3_DEV) \
+                            $(LIBNL_ROUTE3_DEV) $(LIBNL_NF3_DEV) \
+			    $(LIBNL_CLI_DEV)
+$(LIBSWSSCOMMON)_RDEPENDS += $(LIBHIREDIS) $(LIBNL3) $(LIBNL_GENL3) \
+                             $(LIBNL_ROUTE3) $(LIBNL_NF3) $(LIBNL_CLI)
+SONIC_DPKG_DEBS += $(LIBSWSSCOMMON)
+
+LIBSWSSCOMMON_DEV = libswsscommon-dev_1.0.0_amd64.deb
+$(eval $(call add_derived_package,$(LIBSWSSCOMMON),$(LIBSWSSCOMMON_DEV)))
+```
+First we define our package swsscommon.  
+Then we secify **SRC_PATH** (path to sources),
+**DEPENDS** (build dependencies),
+and **RDEPENDS** (runtime dependencies for docker installation).  
+Then we add our target to SONIC_DPKG_DEBS target group.  
+At the end we define a dev package for swsscommon and make it derived from main one.
+Using **add_derived_package** just makes a deep copy of package's metadata, so that we don't have to repeat ourselves.  
+
+### Target groups
+**TODO**  
+## Tips & Tricks
+**TODO**