Don't use -p flag when copying loader.efi to msdosfs; this fixes the 'cdrom' target when world was built by user, not root (i.e. the file in obj/ is owned by user). Otherwise it fails like below:
sh /usr/src/release/amd64/mkisoimages.sh -b 13_0_CURRENT_amd64_CD disc1.iso disc1 200+0 records in 200+0 records out 819200 bytes transferred in 0.003921 secs (208939883 bytes/sec) newfs_msdos: cannot get number of sectors per track: Operation not supported newfs_msdos: cannot get number of heads: Operation not supported /dev/md0: 1557 sectors in 1557 FAT12 clusters (512 bytes/cluster) BytesPerSec=512 SecPerClust=1 ResSectors=1 FATs=2 RootDirEnts=512 Sectors=1600 Media=0xf8 FATsecs=5 SecPerTrack=63 Heads=1 HiddenSecs=0 cp: chown: efi/efi/boot/bootx64.efi: Invalid argument *** Error code 1 Stop. make: stopped in /usr/src/release