uboot-extlinux-config.bbclass: invalidate do_create_extlinux_config on changes to...

uboot-extlinux-config.bbclass: invalidate do_create_extlinux_config on changes to UBOOT_EXTLINUX_FDTOVERLAYS:x

do_create_extlinux_config is using a bit of an odd mechanism which
doesn't work well with sstate cache invalidation.

BitBake will detect changes to UBOOT_EXTLINUX_FDTOVERLAYS because it's
explicitly mentioned in the task, but it'll miss changes to
UBOOT_EXTLINUX_FDTOVERLAYS:label because this OVERRIDES is set within
the task, so the value of UBOOT_EXTLINUX_FDTOVERLAYS for the label
OVERRIDES will only ever change from within the task, while it is
running, much later than during parsing.

For that to work properly, we need to add the entire variable (including
the OVERRIDES part) to the vardeps varflag of the task so that its value
is monitored. This is already done for all possible label variables but
FDTOVERLAYS was forgotten.

Fixes: 3ac21b32b5f5 ("uboot-extlinux-config.bbclass: add support for DTBOs")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>