The board support for an architecture always starts with Qemu support,
this is what each linux_defconfig at the very least sets up. Then
each $BR2_ARCH has additional BSPs, e.g., Banana Pi BPI-R3.
The board/ directory is matched with the configs/*_defconfigs and
the only execption is board/common/, which holds all shared files for
Infix builds.
Each board/$BR2_ARCH/ can then have vendor/product sub-directories
for the BSPs which may contain "fixups" to the base kernel config and
any additional device tree files that should be included as well.
To rebuild a board-specific package, e.g. NanoPi R2S:
make friendlyarm-nanopi-r2s-rebuild all