Documentation: add start of documentation of the build system

Change-Id: Ic4a4b4d71852bfe0b1fc52373e88d0a53b145844
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/16150
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Documentation/build_system.md

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+# The coreboot build system
+(this document is still incomplete and will be filled in over time)
+
+## General operation
+The coreboot build system is based on GNU make but extends it significantly
+to the point of providing its own custom language.
+The overhead of learning this new syntax is (hopefully) offset by its lower
+complexity.
+
+The build system is defined in the toplevel `Makefile` and `toolchain.inc`
+and is supposed to be generic (and is in fact used with a number of other
+projects).  Project specific configuration should reside in files called
+`Makefile.inc`.
+
+In general, the build system provides a number of "classes" that describe
+various parts of the build. These cover the various build targets in coreboot
+such as the stages, subdirectories with more source code, and the general
+addition of files.
+
+Each class has a name (eg. `romstage`, `subdirs`, `cbfs-files`) and is used
+by filling in a variable of that name followed by `-y` (eg. `romstage-y`,
+`subdirs-y`, `cbfs-files-y`).
+The `-y` suffix allows a simple interaction with our Kconfig build
+configuration system: Kconfig options are available as variables starting
+with a `CONFIG_` prefix and boolean options contain `y`, `n` or are empty.
+
+This allows `class-$(CONFIG_FOO) += bar` to conditionally add `bar` to
+`class` depending on the choice for `FOO`.
+
+## classes
+Classes can be defined as required. `subdirs` is handled internally since
+it's parsed per subdirectory to add further directories to the rule set.
+
+TODO: explain how to create new classes and how to evaluate them.
+
+### subdirs
+`subdirs` contains subdirectories (relative to the current directory) that
+should also be handled by the build system. The build system expects these
+directories to contain a file called `Makefile.inc`.
+
+Subdirectories are not read at the point where the `subdirs` statement
+resides but later, after the current directory is handled (and potentially
+others, too).
+
+### cbfs-files
+This class is used to add files to the final CBFS image. Since several more
+options need to be maintained than can comfortably fit in that single
+variable, additional variables are used.
+
+`cbfs-files-y` contains the file name used in the CBFS image (called `foo`
+here). Additional options are added in `foo-$(option)` variables. The
+supported options are:
+
+*  `file`: The on-disk file to add as `foo` (required)
+*  `type`: The file type. Can be `raw`, `stage`, `payload`, and `flat-binary`
+   (required)
+*  `compression`: Can be `none` or `lzma` (default: none)
+*  `position`: An absolute position constraint for the placement of the file
+   (default: none)
+*  `align`: Minimum alignment for the file (default: none)
+*  `options`: Additional cbfstool options (default: none)
+
+`position` and `align` are mutually exclusive.
+
+#### FMAP region support
+With the addition of FMAP flash partitioning support to coreboot, there was a
+need to extend the specification of files to provide more precise control
+which regions should contain which files, and even change some flags based on
+the region.
+
+Since FMAP policies depend on features using FMAP, that's kept separate from
+the cbfs-files class.
+
+The `position` and `align` options for file `foo` can be overwritten for a
+region `REGION` using `foo-REGION-position` and `foo-REGION-align`.
+
+The regions that each file should end in can be defined by overriding a
+function called `regions-for-file` that's called as
+`$(call regions-for-file,$(filename))` and should return a comma-separated
+list of regions, such as `REGION1,REGION2,REGION3`.
+
+The default implementation just returns `COREBOOT` (the default region) for
+all files.
+
+vboot provides its own implementation of `regions-for-file` that can be used
+as reference in `src/vboot/Makefile.inc`.