run the uboot binary through mkknlimg:
mkknlimg --dtok u-boot.bin uboot.bin

U-Boot 2015.07-dirty (Aug 04 2015 - 15:44:23 +0200)

DRAM:  944 MiB
WARNING: Caches not enabled
RPI 2 Model B
MMC:   bcm2835_sdhci: 0
reading uboot.env
In:    serial
Out:   serial
Err:   serial
Net:   Net Initialization Skipped
No ethernet found.
U-Boot> fdt addr 0x100
U-Boot> fdt get value args /chosen bootargs
U-Boot> printenv
args=dma.dmachans=0x7f35 bcm2708_fb.fbwidth=656 bcm2708_fb.fbheight=416 bcm2709.boardrev=0xa01041 bcm2709.serial=0x8b04db1c smsc95xx.macaddr=B8:27:EB:04:DB:1C bcm2708_fb.fbswap=1 bcm2709.disk_led_gpio=47 bcm2709.disk_led_active_low=0 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x3dc00000 vc_mem.mem_size=0x3f000000  elevator=deadline
fdtaddr=256
fdtfile=bcm2836-rpi-2-b.dtb
foo=bar
usbethaddr=b8:27:eb:04:db:1c

Environment size: 435/131068 bytes
U-Boot>