For 2.1, configuring the "minimal webapp" is greatly simplified. In short, read the INSTALL file, configure the build to include the desired blocks, and do a clean build.
For building a minimal webapp for 2.0, see CreateMinimalWebapp
http://cocoon.apache.org/2.1/installing/#Building+Cocoon (section on Blocks)
BlockDescriptions -- describing blocks you may/may not want.
http://cocoon.apache.org/2.1/installing/jars.html -- jar descriptions