FROMLIST: tools/bootconfig: Fix a build error accroding to undefined fallthrough

Since the "fallthrough" is defined only in the kernel, building
lib/bootconfig.c as a part of user-space tools causes a build
error.

Add a dummy fallthrough to avoid the build error.

Picking this up FROMLIST to get the fix ASAP. It is likely to be
accepted upstream.

Signed-off-by: Devin Moore <devinmoore@google.com>
Link: https://lore.kernel.org/lkml/162262192121.264090.6540508908529705156.stgit@devnote2/


Bug: 183237066
Cc: stable@vger.kernel.org
Fixes: 4c1ca831 ("Revert "lib: Revert use of fallthrough pseudo-keyword in lib/"")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Change-Id: I624b42f8d20f57188768f4db3b68c23b0918cbef