Same problem with EX2300-C.
It was not a bright decision to put bootloader vars into a bootloader script file.
To change currdev var you have to get to loader> prompt, and even inserting valid USB stick doesn't help.
To get to loader prompt,
setenv loaderdev usb1:0
boot
From loader prompt tftp --format is failing with
cannot open package (error 60)
boot -s also failing (Enter full pathname of shell or RETURN for /bin/sh: ), supposedly something wrong with chroot.
UPDATE: All 15.1X53-D59 images(tftp, USB install package+media) failed just as 15.1X53-D50, but 15.1X53-D58 USB install succeeded, however after install again a loaderdev override to usb0:0 was required.
@Juniper please bring back normal QA/release testing.