My specific mx6cuboxi device is the cubox-i2ex.

Wrote image to sdcard as follows:

arm-image-installer --image=Fedora-Minimal-armhfp-27_Beta-1.5-sda.raw.xz \
  --target=mx6cuboxi --media=/dev/sdX --resizefs --addconsole

When device is power, boot is stuck in a perpetual loop that prints the following on the serial console:

U-Boot SPL 2017.09-rc2 (Aug 25 2017 - 18:06:14)
Trying to boot from MMC1


On my working Fedora system, I installed uboot-images-armv7, which came as version 2016.09.01-2.

I manually installed the SPL, and u-boot.img in the manner that arm-image-installer does.

dd if=/usr/share/uboot/mx6cuboxi/SPL of=/dev/sdX bs=1k seek=1
dd if=/usr/share/uboot/mx6cuboxi/u-boot.img of=/dev/sdX bs=1k seek=69

The previously written image of F27 Beta then successfully boots:

U-Boot SPL 2016.09.01 (Oct 19 2016 - 14:40:29)
Trying to boot from MMC1


U-Boot 2016.09.01 (Oct 19 2016 - 14:40:29 +0000)

CPU:   Freescale i.MX6D rev1.5 996 MHz (running at 792 MHz)
CPU:   Extended Commercial temperature grade (-20C to 105C) at 15C
Reset cause: POR
Board: MX6 Cubox-i
DRAM:  1 GiB
MMC:   FSL_SDHC: 0
[ ... continues to boot normally, as desired ... ]

Not sure what is wrong with U-Boot as present on the F27 Beta image, which reports itself to be version 2017.09-rc2.

I hope this report is helpful and you are able to find a fruitful solution.