Update u-boot submodule revision. - arm-sdk - os build toolkit for various embedded devices

commit 6518c5c494502d710a8f06e9e1cec48c211b96c1
parent ea304c82796c0a2b4693c4573b6494fec70b1bd1
Author: parazyd <parazyd@dyne.org>
Date:   Mon,  6 Jul 2020 01:07:30 +0200

Update u-boot submodule revision.

	arm-sdk os build toolkit for various embedded devices
	git clone https://git.parazyd.org/arm-sdk
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