As currently setup, with the production BeagleBone Black's (BBB), it's
really not an "option" as, it "will" boot from eMMC by default..
1: On the BBB: the BootROM searches the eMMC before the microSD
2: Out of the box, each BBB has the bootloader/system already flashed to eMMC
So for new "users": by default the BBB boots from eMMC, but it can be
an "option" to boot from microSD (if they were to reformat/remove the
MLO from the eMMC)..
Regards,