.gitmodules: update vboot submodule to track branch=main

vboot has been updated to track main branch, however the
.gitmodules defaults to master branch following the
coreboot default. This impacts the rebase of submodule
git submodule update --remote --rebase 3rdparty/vboot/

With this change the rebase to latest commit is successful

Signed-off-by: Balaji Manigandan B <balaji.manigandan@intel.com>
Change-Id: I7713aecdec43a5d5623ef81803ac0fc02ce14070
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52664
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/.gitmodules b/.gitmodules
index 4350a2c..d798fd5 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -9,6 +9,7 @@
 [submodule "vboot"]
 	path = 3rdparty/vboot
 	url = ../vboot.git
+	branch = main
 [submodule "arm-trusted-firmware"]
 	path = 3rdparty/arm-trusted-firmware
 	url = ../arm-trusted-firmware.git