In reference to the master branch:
d1388f36 ("treewide U-boot: enable
parallel building")

This fixes:
Makefile:1452: *** 'BL33=/foo/build/staging_dir/target-aarch64_cortex-a53_musl/root-mvebu/usr/share/mox/u-boot.bin' was specified, but '/foo/build/staging_dir/target-aarch64_cortex-a53_musl/root-mvebu/usr/share/mox/u-boot.bin' does not exist.  Stop.

uboot is now built directly from upstream, the package name is
mox-uboot-uboot and it's build only, so users can not install this
package. It's used to build a new package called mox-a53-firmware that
combines uboot with trusted firmware and provides the uboot-devel binary
like uboot-mox used to.