grub2: fix CVE-2024-56738

grub2: fix CVE-2024-56738

Backport an algorithmic change to grub_crypto_memcmp() so that it
completes in constant time and thus isn't susceptible to side-channel
attacks.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 30a1cc225a2bd5d044bf608d863a67df3f9c03be)
Signed-off-by: Shubham Pushpkar <spushpka@cisco.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>