This variable is part of uboot-sign but not documented.
(From yocto-docs rev: 999e81f17bf60e187e709368ede3965df19bee59)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 05eb461cb1da76ad9cbaf634da7f47447b3f6765)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>