pxelinux: use a table-driven backoff

pxelinux: use a table-driven backoff

Use a table-driven backoff so we can provide a backoff which doesn't
increase as sharply (by integral powers of two) as before.  Rather
than worrying about doing fractional multiplication, simply provide a
backoff table.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>