IMHO the default u-boot for version of the image SD_USB not needed. In normal state of run of the images with SD and USB VIM\VIM2 must be used the boot loader from the internal memory (eMMC or the SPI) in the multi-boot. If someone will need to make the SD card bootable (this is clearly not regular users), he can record the desired option with u-boot on the SD card.
You can further release the finished versions of the images of the “recovery system” specifically for VIM and VIM2 for separately (recording on the SD card). In which will be installed the right version of the u-boot. These images are a set of tools (scripts) for fast system recovery, backup, etc. the Logic behind the use of such a “recovery image”. The user downloaded the image , recorded on the SD card, hooked up to VIM, and using the M-register, has launched a system (regardless of the state of the eMMC and boot loader in it). You can add the autoscript which when executed will automatically restore the firmware in eMMC to default (factory) state.