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This appears to be historical from when the toolchain was in meta-linaro.

It isn't needed anymore, there's one bbappend in meta-arm-toolchain for
grub which is part of oe-core, so will never be dangling.

This variable has a global effect, so leaving it in here has a negative
impact on users.
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>