To cross-compile, pass CC=target-gcc and OS=target-os if the target OS differs from your host to make. You can configure the build with use flags
The bootrom.bin file is a user-provided raw binary, loaded at 0x80000000 address where it starts execution, and device.dtb is a DTB file containing description of the machine. You can pass -image=rootfs.img to mount a raw partition image as a flash drive.