These are the steps that I followed:

• Create folder b2qt

• Launch command: repo init -u git://code.qt.io/yocto/boot2qt-manifest -m v5.14.2.xml

• Launch command: repo sync (with this command all the meta-layer folders are created)

• Modify the manifest file adding the sunxi layer (the manifest file is contained in b2qt/sources/meta-boot2qt/scripts/manifest.xml not in conf/manifest.xml as explained in the guide):
  <!-- Banana Pi -->
  <project name="meta-sunxi"
  remote="sunxi"
  revision="warrior"
  path="sources/meta-sunxi"/>

• Add a sample bblayers in b2qt/sources/meta-boot2qt/meta-boot2qt-distro/conf named bblayers.conf.sunxi.sample

• Add a configuration file in b2qt/sources/meta-boot2qt/meta-boot2qt-distro/conf/distro/include folder for the machines that I'm going to build, in my case bananapi.conf (the problem with this file is that there is a banana.conf in meta-sunxi, but it has several includes that it's hard to have in a different folder, need help here)

• I launch the command repo init -u /home/davide/b2qt/sources/meta-sunxi -m /home/davide/b2qt/sources/meta-boot2qt/scripts/manifest.xml -b warrior
  I get the following error:
  fatal: manifest '/home/davide/b2qt/sources/meta-boot2qt/scripts/manifest.xml' not available
  fatal: remote sunxi not defined in /home/davide/b2qt/sources/meta-boot2qt/scripts/manifest.xml

Thank you in advanced.

Best Regards,
Davide