Fix a bug where the submenu timeout is corrupted by an incorrect sed
command.
In addition, update handling of 0 second timeout to set the timeout in
grub.cfg to 0.001, to avoid issues on some systems with timeout=0.
Change-Id: I0683f39eb906da512cacc77bde28f8420a73a001
Closes-Bug: 1856764
Signed-off-by: Don Penney <don.penney@windriver.com>