Build system changes for 4.2.3

This page describes changes to the 4.2.3 build system.

Build system documentation has been updated and overhauled.
Examples are now compiled automatically in non-singleexec builds. They are not installed.
qtopiamake -debug when the SDK is release no longer strips the binary at install time.
content_installer -remove lets you remove content items from the database.
The handling of .directory files and the application categories has been corrected.
Quirks in the handling of content items by make install vs make packages has been documented.
make sdk removes the SDK directory before installing to ensure stale files are not present. This matches the behavior of make install.