lib/cmdline.c: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

lib/cmdline.c:137:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
lib/cmdline.c:140:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
lib/cmdline.c:143:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
lib/cmdline.c:146:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
lib/cmdline.c:149:7: warning: this statement may fall through [-Wimplicit-fallthrough=]

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling -Wimplicit-fallthrough.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>