It seems that some benign operations in U-boot cause it to abort. For example, I can type "printenv", or "printenv bootcmd", but if I type "printenv findfdt" or "printenv uname_boot", I get this:
=> printenv uname_boot
uname_boot=setenv bootdir /boot; setenv bootfile vmlinuz-${uname_r}; if test -e mmc ${bootpart} ${bootdir}/${bootfile}; then echo loading ${bootdir}/${bootfile} ...; run loadimage;setenv fdtdir /boot/dtbs/${uname_r}; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else setenv fdtdir /usr/lib/linux-image-${uname_r}; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else setenv fdtdir /lib/firmware/${uname_r}/device-tree; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else setenv fdtdir /boot/dtb-${uname_r}; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else setenv fdtdir /boot/dtbs; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else setenv fdtdir /boot/dtb; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else setenv fdtdir /boot; if test -e mmc ${bootpart} ${fdtdir}/${fdtfile}; then run loadfdt;else if test -e mmc ${bootpart} ${fdtfile}; then run loadfdt;else echo; echo unable to find [dtb=${fdtfile}] did you name it correctly? ...; run failumsboot;fi;fi;fi;fi;fi;fi;fi;fi; setenv rdfile initrd.img-${uname_r}; if test -e mmc ${bootpart} ${bootdir}/${rdfile}; then echo loading ${bootdir}/${rdfile} ...; run loadrd;if test -n ${uuid}; then setenv mmcroot UUID=${uuid} ro;fi;run mmcargs;echo debug: [${bootargs}] ... ;echo debug: [bootz ${loadaddr} ${rdaddr}:${rdsize} ${fdtaddr}] ... ;bootz ${loadaddr} ${rdaddr}:${rdsize} ${fdtaddr}; else run mmcargs;echo debug: [${bootargs}] ... ;echo debug: [bootz ${loadaddr} - ${fdtaddr}] ... ;bootz ${loadaddr} - ${fdtaddr}; fi;fi;
prefetch abort
pc : [<64746468>] lr : [<64746466>]
reloc pc : [<f00b4403>] lr : [<f00b4401>]
sp : 9ef40d38 ip : 00000000 fp : 00000002
r10: 9ffa4e74 r9 : 9ef40ed8 r8 : 9ef48e30
r7 : 00000001 r6 : 00000000 r5 : 9ef48e34 r4 : 20766e65
r3 : 9ff7a331 r2 : 44e09000 r1 : 00000000 r0 : 0000063c
Flags: nzCv IRQs off FIQs on Mode SVC_32
Resetting CPU ...
resetting ...