Change u-boot submodule URLs to GitLab mirrors

[intrigus-lgtm]

See qemu@a11d184

To continue our GitLab Open Source Program license we need to pass an
automated license check for all repos under qemu-project. While U-Boot
is clearly GPLv2 rather than fight with the automated validation
script just move the mirror across to a separate project.

[intrigus-lgtm]

This is similar to #70 but, this is the "proper" fix that the qemu devs themselves did.

[McSinyx]

Can we please remove those submodules? I don't think AFL++ runs QEMU in system emulation mode. See also GH-64, but I figured there'd be more progress to do it one-by-one then.