I figured out how to deal with u-boot-fslc. The newer versions of u-boot no longer use boards.cfg. Here are the following steps I had to do to perform the equivalent tasks in the newer version of u-boot:
1. edit arch/arm/cpu/armv7/mx6/Kconfig and add my custom target, following the same syntax as the other targets. At the end of the file, add the path to another Kconfig file located where the source files for your target can be found (e.g. board/<vendor>/<target>/Kconfig).
2. create and edit board/<vendor>/<target>/Kconfig. Follow similar convention to Kconfig from a reference board.
3. create and edit board/<vendor>/<target>/Makefile. Follow similar convention to Makefile from a reference board.
4. create and edit board/<vendor>/<target>/customboard.c (customboard.c being named after your custom board and matching the name of the .o file from the Makefile above). This contains all init code, etc.
5. create and edit configs/customboard_defconfig. Follow similar convention to _defconfig file from a reference board.
6. create and edit include/configs/customboard.h. This contains configuration settings for the board.
7. You should be ready to make now. Run:
make distclean
make customboard_defconfig
make menuconfig
go through target settings and other settings and make sure everything is correct
make
This is not a general guide for all custom boards, but is a good starting point for porting the BSP. I hope that in the next version of the Official BSP, the BSP porting guide is updated to reflect the version of u-boot that is actually pulled during build.