Define oob_config_dir env var and interpolation

Path references in the config file can now refer to %(oob_config_dir)s
which resolves to the directory where the config file resides.

Additionally, scripts will find $oob_config_dir defined in the environment.

For example this will find a custom script in the same dir as the config
file:

  [custom_scripts]
  custom_script_1=%(oob_config_dir)s/myscript.sh

In a custom script, this will apply a patch in the same directory as the
config file:

	patch $INSTALL_DIR/etc/foo < $oob_config_dir/foo.patch

[dsd: minor tweak, add documentation]

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/README b/doc/README
index 4cac113..ceb7209 100644
--- a/doc/README
+++ b/doc/README
@@ -89,6 +89,15 @@ the first one are indented, e.g.:
 		but lines 2 through 4 must be indented
 	option2=foo
 
+Interpolation (basic assignment and use of variables) can be used, according
+to Python's ConfigParser documentation.
+
+Default variables automatically provided:
+
+oob_config_dir
+
+	The directory where the currently selected config file resides
+
 In general, each section that can exist in the configuration file
 corresponds to a specific module.  The section name is the same as the
 module name.  The options that can exist in these sections are