env: Include bootdelay in environment if negative

env: Include bootdelay in environment if negative

The test for (CONFIG_BOOTDELAY >= 0) has been in U-Boot since the
beginning, but the meaning of it has changed over time. Allow the
default to be set for any value, including -ve ones. This allows
(for example) CONFIG_ENV_IS_NOWHERE to have values for bootdelay in
its compiled in environment.

The only thing this changes is where the default for bootdelay can be
fetched from; before this change you get a compiled in default, after
you'll pull it from the default value in the environment, but both values
will be the same. Also if there's a value set in the environment then
that will take precedence (as before).

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>