Not all board variants are represented with a specific defconfig in
mainline U-Boot. Here a small documentation on how to generate U-Boot
images for all other board variants, available via different dts
files and defconfigs.

Use a different dts than in the defconfig:

make DEVICE_TREE=cn9131-db-B

Use a different boot device (e.g. MMC or NAND instead of SPI NOR):

For MMC, please make the following changes to the defconfig via
e.g. "make menuconfig":
Remove CONFIG_MVEBU_SPI_BOOT
Select CONFIG_MVEBU_MMC_BOOT
Remove CONFIG_ENV_IS_IN_SPI_FLASH
Select CONFIG_ENV_IS_IN_MMC

For NAND, please make the following changes to the defconfig via
e.g. "make menuconfig":
Remove CONFIG_MVEBU_SPI_BOOT
Select CONFIG_MVEBU_NAND_BOOT
Remove CONFIG_ENV_IS_IN_SPI_FLASH
Select CONFIG_ENV_IS_IN_NAND