Pull up following revision(s) (requested by jakllsch in ticket #831):

sys/arch/i386/stand/efiboot/eficons.c: revision 1.6

During awaitkey(), move the cursor back to the beginning of the
countdown with '\b' only after waiting.

Some UEFI implementations have a destructive/erasing backspace,
and will not show the countdown if we move the cursor back before the
wait.

My ThinkPad T430 w/ Lenovo's UEFI exhibited this whiteout-backspace,
while my Gigabyte Brix GB-BXBT-2807 did not.

Should fix PR #53292.

(martin)