Download the latest version of U-Boot and extract the files to a working folder:
wget ftp://ftp.denx.de/pub/u-boot/u-boot-latest.tar.bz2 tar -xjf u-boot-latest.tar.bz2 cd u-boot-2015.07
Now cross compile U-Boot using the am335x BeagleBone Black configuration:
make CROSS_COMPILE=arm-linux-gnueabi- distclean make CROSS_COMPILE=arm-linux-gnueabi- am335x_boneblack_config make CROSS_COMPILE=arm-linux-gnueabi-
Compiling should conclude with output similar to below showing that an image of MLO - the first stage uBoot Bootloader is generated.
LD spl/u-boot-spl OBJCOPY spl/u-boot-spl.bin MKIMAGE MLO CFG spl/u-boot-spl.cfg
Scrolling up the output messages, you should find where the second stage U-Boot bootloader (u-boot.img) is generated.
LDS u-boot.lds LD u-boot OBJCOPY u-boot.bin MKIMAGE u-boot.img OBJCOPY u-boot.srec CFG u-boot.cfg
If desired, you can test the second stage bootloader by loading it to RAM:
setenv ipaddr 192.168.0.250 setenv serverip 192.168.0.251 tftp 0x80800000 u-boot.bin go 0x80800000