I submitted a patch for "flash-kernel" to Ubuntu to solve this problem:
Kernel /boot/vmlinuz-3.4.0-1-linaro-lt-omap does not match your subarchitecture omap4, therefore not writing it to flash.
The problem has been reported as
I submitted a patch, but for the patch to be included in a "stable release update" (aka SRU), it needs to be tested on a BeagleBoard-xM.
If you are running Ubuntu on a BeagleBoard-xM, will you help me test this patch?