I have updated the internal eMMC flash and sdcard images for BeagleBone Black. Most changes are to make U-Boot work better with fastboot, especially to make it possible to enter fastboot without having to type "fastboot" at a U-Boot prompt and so avoid the need to have a serial cable. Also, I have increased the sizes of the system and userdata partitions so that you can install more apps. I am still using Android KitKat 4.4.4 and TI Linux 3.2 kernel with SGX accelerated graphics libraries.
Many Android bootloaders implement the fastboot protocol for loading and flashing images to internal memory (there is some background information about how Android boots and boot image formats in these slides: http://www.slideshare.net/chrissimmonds/android-bootslides20). Fastboot is a simple USB protocol using ASCII text strings for commands and responses. In recent versions of the AOSP you can find the specification in system/core/fastboot/fastboot_protocol.txt.