Remove state tracking, use KAS_WORK_DIR env variable

Config.has_changed didn't work reliably so far, so the state cache was
only allowing to associate a work directory with configuration file.
However this was rather unintuitive, specifically when moving the work
dir to a different home - nothing told the user about the persistent
association.

We therefore agreed to remove the state cache for now. It can be
reintroduced at any time once a consistent usage model exists.

To establish a relationship to a work directory inside a shell session,
kas now evaluates the KAS_WORK_DIR environment variable.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

README.md

@@ -29,7 +29,6 @@ Dependencies & installation
 This projects depends on
 
 - Python 3
-- configparser
 - PyYAML
 
 If you need Python 2 support consider sending patches. The most
@@ -67,6 +66,10 @@ manually, e.g.
 $ kas shell /path/to/kas-project.yml -c 'bitbake dosfsutils-native'
 ```
 
+kas will place downloads and build artifacts under the current directory when
+being invoked. You can specify a different location via the environment variable
+`KAS_WORK_DIR`.
+
 
 Use Cases
 ---------