We placed a wire across the J5 jumper (ties PMIC POWERHOLD to LDO_VRTC) to permit the BB-X15 to boot up after power loss without having to press the power button. Normally without the jumper, the PMIC (power management chip) shuts the am5728 down within a fixed time window after powerup. Placing a wire across the J5 jumper allows the PMIC to keep the Sitara 5728 powered on all the time, thus not requiring the user to press the power or RESET buttons to boot the BB-X15. Normal mode of operation after pressing the power button with a valid u-boot present is for u-boot to send a message to the PMIC to remain powered on. But with the jumper populated, this isn't needed to allow the PMIC to keep the 5728 powered.
Incidentally, if you do connect JTAG with CCS, TI provides a set of GEL (target initialization) files which send the "stay powered" message to the PMIC. I believe the GEL files run when the user attempts to connect to the A15 core(s) via CCS. But you have to quickly connect to the A15 core(s) after pressing the power button before the PMIC shuts everything down again.