legacy_sdcard_do_upgrade needs partx after a full image write, but only
some targets listed it in DEFAULT_PACKAGES, where it could be deselected.
Select it from the feature and add it to DEFAULT_PACKAGES for the
ImageBuilder, which also covers qoriq, sifiveu and mvebu/cortexa9.
Signed-off-by: John Audia <therealgraysky@proton.me>