Improve MOX U-boot Makefiles

This silences warning: cc1: note: someone does not honour COPTS correctly, passed 0 times

As OpenWrt does it, too.

changed milestone to %Turris OS 6.0

requested review from @mvasilek

assigned to @mvasilek

mentioned in merge request build!559 (merged)

added 2 commits

• 1bd4a888 - mox-uboot: removed it as it was superseded by uboot-mvebu
• 58884ce6 - mox-a53-firmware: use U-boot image from uboot-mvebu

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mentioned in merge request !968 (merged)

approved this merge request

assigned to @jschlehofer and unassigned @mvasilek

resolved all threads

added 13 commits

• 58884ce6...2e2cde37 - 9 commits from branch develop
• 69290fd6 - mox-a53-firmware: supress warning and use proper variable
• 34ef481a - mox-uboot: use proper variables for HOST{CF,LD}FLAGS
• 5ca4a228 - mox-uboot: removed it as it was superseded by uboot-mvebu
• ac41f1d5 - mox-a53-firmware: use U-boot image from uboot-mvebu

Compare with previous version

merged