uboot-extlinux-config.bbclass: fix missed override syntax migration

uboot-extlinux-config.bbclass: fix missed override syntax migration

uboot-extlinux-config allows to specify multiple "labels" (entries in a
menu, à-la grub) and each of them have their own values for some fields.
Each "base" variable, e.g. UBOOT_EXTLINUX_FDT can be overridden for each
label. This is done via the OVERRIDES mechanism based on the label name,
e.g. UBOOT_EXTLINUX_FDT:linux if linux is a label.

However, OVERRIDES doesn't contain the label globally because it's only
necessary in one task. Therefore, the OVERRIDES itself is modified
within the task. This means that the sigdata will not be told the
dependency on UBOOT_EXTLINUX_FDT:linux, because it cannot know about it.

For this reason, we need to explicitly specify which variables this task
depends on via vardeps varflag for the task.

This was done in the past, but we missed updating it during the override
syntax migration so the cache wouldn't get invalidated if someone
modifies UBOOT_EXTLINUX_FDT:linux from a configuration file or a
bbappend for example.

Let's fix this by migrating it to the new syntax.

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>