kconfig: improvements for builddir config file

Document best practices for per-builddir config file, and add a test covering
loading a config file from the build directory.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/markdown/Kconfig-module.md

@@ -38,6 +38,14 @@ This function loads a kconfig output file and returns a dictionary object.
 file.  Therefore, true boolean values will be represented as the string "y"
 and integer values will have to be converted with `.to_int()`.
 
+Kconfig frontends usually have ".config" as the default name for the
+configuration file.  However, placing the configuration file in the source
+directory limits the user to one configuration per source directory.
+In order to allow separate configurations for each build directory, as is
+the Meson standard, `meson.build` should not hardcode ".config" as the
+argument to `kconfig.load()`, and should instead make the argument to
+`kconfig.load()` a [project build option](Build-options.md).
+
 * The first (and only) argument is the path to the configuration file to
   load (usually ".config").
 

test cases/kconfig/4 load_config builddir/config

@@ -0,0 +1,2 @@
+CONFIG_IS_SET=y
+# CONFIG_NOT_IS_SET is not set

test cases/kconfig/4 load_config builddir/meson.build

@@ -0,0 +1,14 @@
+project('kconfig builddir test')
+
+k = import('unstable-kconfig')
+
+configure_file(input: 'config', output: 'out-config', copy: true)
+conf = k.load(meson.build_root() / 'out-config')
+
+if not conf.has_key('CONFIG_IS_SET')
+  error('Expected CONFIG_IS_SET to be set, but it wasn\'t')
+endif
+
+if conf.has_key('CONFIG_NOT_IS_SET')
+  error('Expected CONFIG_NOT_IS_SET not be set, but it was')
+endif