Update vboot submodule to upstream master

Updating from commit id 9d4053d:
2020-11-20 01:51:08 +0000 - (Revert "Reland: Clean up implicit fall through.")

to commit id 48195e5:
2020-11-24 10:23:45 +0000 - (Makefile: Test for warning flags before using them)

This brings in 3 new commits.

Change-Id: I64f27f346df264cb6eeeb4e3203fcca7d35f7e83
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47906
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Idwer Vollering <vidwer@gmail.com>

3rdparty/vboot

@@ -1 +1 @@
-Subproject commit 9d4053df76c127f625a8571d3ef16e6a063c8de5
+Subproject commit 48195e5878006ac2cf74cb7f02953ab06c68202d