TCB validation logic looks flawed

[deeglaze]

	min, err := kds.ComposeTCBParts(options.MinimumTCB)
	if err != nil {
		return fmt.Errorf("option MinimumTCB error: %v", err)
	}
	if kds.TCBVersion(report.GetCurrentTcb()) < min {
		return fmt.Errorf("firmware's current TCB %x is less than required %x",
			report.GetCurrentTcb(), min)
	}

This treats the TCB_VERSION as a single number instead of an array of lower bounds for each kind of SPL. This means the accidental position of one SPL can mask a version mismatch in a lower order version. For example, [2, 1] is greater than [1, 3], but you don't want to allow a lower SPL for the number on the right because the number on the left is higher.

We should treat the minimum TCB_VERSION as a piecewise minimum.

[deeglaze]

Fixed with PR#59