Improve workaround for do_image_wic race condition

Improve workaround for do_image_wic race condition

This change ensures that no other do_image_* task can be running and
reading the rootfs directory at the same time as do_image_wic is
modifying the /etc/fstab file in the rootfs directory. It can be
removed when the more efficient but more complex fix is accepted
and backported upstream in poky.

Further details are given in the new image_types_wic_workaround.bbclass
file.

This allows us to remove the previous workaround of running wic with the
`--no-fstab-update` argument and so the fstab file in wic images will
now include all partitions.

Bug-AGL: SPEC-3621

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Change-Id: Ibdbed7164c8684f50e917b5c877508d8c968d73b
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25884
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>