apm821xx: explicitly build the rootfs.img.gz target

apm821xx: explicitly build the rootfs.img.gz target

The commit 87b668765e1
("image: when using the new image build code, gzip ext4 images by default")

forced that all targets that select the ext4 as the root filesystem
to always compress the generated rootfs. This is fine, but this method
doesn't not allow to append the metadata on a per-target base.

Therefore this patch changes the rootfs image production rule to generate
the gzip step manually. This way the metadata can be appended at a later
date.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>