Official document:
https://chromium.googlesource.com/chromiumos/docs/+/master/developer_guide.md

Fetch code from Google
$ repo init -u https://chromium.googlesource.com/chromiumos/manifest.git –repo-url https://chromium.googlesource.com/external/repo.git -b stable

1. If you want to clone single library, using
   $ repo sync “src/third_party/libqmi”
   
   If you want to fetch all libraries, using
   $ repo sync -c
   
2. Enter the root of sdk
   $ cd chromite/bin
   
3. Enable virtual machine environment
   $ ./cros_sdk
   
   /***
   cros_sdk:make_chroot: All set up. To enter the chroot, run:
   $ cros_sdk –enter
   
   CAUTION: Do NOT rm -rf the chroot directory; if there are stale bind
   mounts you may end up deleting your source tree too. To unmount and
   delete the chroot cleanly, use:
   $ cros_sdk –delete
   ***/
   
4. Configure cross compiler toolchain
   $ setup_board –board=arm64-generic
   
   if you want to build kernel, set your password.
   // ./set_shared_user_password.sh
   
   Check libraries
   // cros_workon –board=arm64-generic –all list
   
5. Fetch toolchain
   ./build_packages –board=arm64-generic
   
   Some configurations.
   // update_chroot –toolchain_boards arm64-generic –usepkg
   //loman add –workon chromiumos/third_party/libqmi
   //cros_workon –board=arm64-generic start libqmi
   //cros_workon_make –board=arm64-generic libqmi
   
   
6. Start to build pre-build libraries and your target.
   // https://chromium.googlesource.com/chromiumos/docs/+/master/portage/ebuild_faq.md#How-does-the-cross-compiling-setup-work
   
   $ emerge-arm64-generic libqmi