wic: Add option to not change fstab

wic: Add option to not change fstab

Create an option to wic doesn't change fstab file, the final
fstab file will be same that in rootfs and wic doesn't update
file, e.g adding a new mount point.

Users can control the fstab file content in base-files recipe.
This is useful if you want to only create an partition but not
add fstab mount point or add new mount point using label e.g:

LABEL=recovery /recovery auto defaults 0  1

(From OE-Core rev: 00420ec42140c1b752132bda190dede85756d157)

Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>