sd-boot: Draw custom edit cursor

sd-boot: Draw custom edit cursor

Firmware likes to draw the EFI provided cursor in a weird way that
makes it invisible sometimes. This is even more likely to happen
if unusual colors are picked. It also fails to draw attention to the
user by being very small and not blinking.

Additionally, to make it more clear that we are in edit mode, we
now default to inverting the general default color and use that for
our line edit.

Fixes: #19301