[wic] race condition: do not update the fstab

[wic] race condition: do not update the fstab

do_image_wic will run in parallel with other jobs
in the current codebase of OE. Then these other
jobs might get the wrong content for
/etc/fstab that is modified by do_image_wic.

To prevent this behaviour, disable the modification in wic.

NOTE: This might have consequences on custom or complex
wks files. Thus allow the user to override this.

Bug-AGL: SPEC-3621
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Change-Id: I03cb589fb592c1d7e0ccfff54535b91ba73974be
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25412
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: Scott Murray <scott.murray@konsulko.com>