Where to find U-Boot source code in Digi Embedded Yocto

After installing Yocto, you first need to build/bitbake a project for your platform.

Then just search in your $/(WORKSPACE)/$(PROJECT) directory,
for specific files which are typically in u-boot source directory during compilation, e.g.:
# find ~/workspace/ccimx6sbc/tmp -name cmd_bootm.\*

It will point you to:
tmp/work/ccimx6sbc-dey-linux-gnueabi/u-boot-dey/2013.04-r0/git/common/cmd_bootm.o
tmp/work/ccimx6sbc-dey-linux-gnueabi/u-boot-dey/2013.04-r0/git/common/cmd_bootm.c

So, you may modify U-Boot sources in:
tmp/work/ccimx6sbc-dey-linux-gnueabi/u-boot-dey/2013.04-r0/git/* and subdirs
to get them compiled during next bitbake