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.
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.

This makes cdboot working for MacBook Pro with Boot Camp.

MFC after:	1 day

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