I’m currently running V2.7 (QT 5.7.1). I added qtserialbus and qtserialport to local conf.
Although qtserialbus exist in the meta-qt5 recipie and i can confirm it is properly compiled (I ran bitbake -v qtserialbus) it is not deployed.
oe-core/build/buildhistory/sdk/angstrom-glibc-x86_64-armv7at2hf-neon/meta-toolchain-qt5/target/installed-packages.txt does not contain qtserialbus and QT is complaining about the unknown package.