I have a scheme code like
(define rpi-boot-partition
(size (* 128 (expt 2 20)))
(initializer (gexp initialize-efi-partition))))
(define rpi-root-partition
(initializer (gexp initialize-root-partition))))
(define raspberry-pi-image
(partitions (list rpi-boot-partition rpi-root-partition))))
$ fdisk -l /gnu/store/hnsfs5a161f5hvymg7ar541qrml62yvv-raspberry-pi-barebones-raw-image
Device Boot Start End Sectors Size Id Type
/gnu/store/hnsfs5a161f5hvymg7ar541qrml62yvv-raspberry-pi-barebones-raw-image1 1 262144 262144 128M 83 Linux
/gnu/store/hnsfs5a161f5hvymg7ar541qrml62yvv-raspberry-pi-barebones-raw-image2 262145 3583688 3321544 1.6G 83 Linux
No boot flag is set on the first partition. The file system seems to be incorrect (ext* instead of vfat)