When enabling A20, put upper limit on amount of time we wait for the

When enabling A20, put upper limit on amount of time we wait for the
keyboard controller to get ready (65K x ISA access time, visually
around 1 second).  If we have wait more than that amount it's likely
that the hardware is a legacy-free one and simply doesn't have keyboard
controller and doesn't require enabling A20 at all.

Taken-from: FreeBSD (with modifications suggested by Matt)

 relavant commit logs(and discussion):
  http://docs.FreeBSD.org/cgi/mid.cgi?200604110439.k3B4dTOD072774
  http://docs.FreeBSD.org/cgi/mid.cgi?200604112053.k3BKrnC1053262
  http://docs.FreeBSD.org/cgi/mid.cgi?200604260605.k3Q65GZZ095546