Update vboot submodule to upstream main

Updating from commit id a975eed306:
   2kernel.c: check display request in vb2api_kernel_phase2
to commit id 18cb85b52d:
   2load_kernel.c: Expose load kernel as vb2_api

Signed-off-by: Subrata Banik <subratabanik@google.com>
Change-Id: I58c5d54723683cef51e416fc6f58da000507fbcc
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66269
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

3rdparty/vboot

@@ -1 +1 @@
-Subproject commit a975eed306e16947c30b48ccd25ab67a37295742
+Subproject commit 18cb85b52d689b4cfbd49c6384ae2088b78af1e0