Add the proper loader.efi to each of the ESPs we create. zfsboot creates
a list of extra ESPs to populate. For both UFS and ZFS, the first ESP is
mounted and we copy the files over (using the same heuristic for the
platform default file). zfsboot-noted ESPs will get copies as well. We
don't have to do anything for UFS since we don't support installing into
a gmirror, but if we ever do, we can use this mechanism.
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