I have a vanilla build of gitlab.com/buildroot.org/buildroot.git @ b200632b1ab for raspberrypi0w_defconfig
which seems fine directly booting the kernel using the stock Broadcom binary bootloader.  However
changing the Buildroot bootloader config to BR2_TARGET_UBOOT=y, several issues appear:

1. While output/images/u-boot.bin is generated, it is not present in output/images/boot.vfat nor does the file /boot/config.txt reflect the expected launch of u-boot with "kernel=u-boot.bin" but rather remains set to directly boot the kernel zImage.
2. Implementing #1 manually (which is the drill for grafting a standalone u-boot into an existing PI install) does not result in u-boot coming up out of reset.
3. If I take the same Buildroot built u-boot and install it (per #2) in a creaky Raspbian 9 install, u-boot functions just fine.
4. If I take a git.denx.de/u-boot.git standalome build of u-boot.bin which known to work on the above Raspbian 9, it similarly fails in the Buildroot generated install.
   

So a few thoughts:

1. I may be missing some configuration step given the seeming lack of image preparation in #1.
2. Otherwise #1 may imply full Buildroot + u-boot integration remains a WIP, at least for a Raspberry Pi Zero W.
3. The actual root cause may be an incompatible holy binary Broadcom firmware package being selected as the only other source of root cause appears to be the /boot installed bootcode.bin and/or start.elf.   I do find a limited set of config knobs in the form "BR2_PACKAGE_RPI_FIRMWARE*" but at this point would like to have some sense of where in this problem space I may be.
   

For reference, this is the Buildroot .config content change from a successful direct boot of the kernel zImage to the selection of u-boot as an intermediate boot loader:

dot-config-default                               ?  .config                                        
---------------------------------------------------------------------------------------------------
# BR2_TARGET_UBOOT is not set                    |  BR2_TARGET_UBOOT=y                             
                                                 >  BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES=""      
                                                 >  BR2_TARGET_UBOOT_PATCH=""                      
                                                 >  BR2_TARGET_UBOOT_CUSTOM_DTS_PATH=""            
                                                 >  BR2_TARGET_UBOOT_FORMAT_BIN=y                  
                                                 >  BR2_TARGET_UBOOT_VERSION="2024.01"             
                                                 >  BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y        
                                                 >  BR2_TARGET_UBOOT_BOARD_DEFCONFIG="rpi_0_w"     
                                                 >  BR2_TARGET_UBOOT_LATEST_VERSION=y              
                                                 >  BR2_TARGET_UBOOT_USE_DEFCONFIG=y               
                                                 >  BR2_TARGET_UBOOT_CUSTOM_MAKEOPTS="" 
Any illumination here much appreciated.