I am trying to change the bootloader autoboot options, so that the CONFIG_AUTOBOOT_STOP_STR is "stop" rather than " ". The reason I am doing this is because I suspect the Beaglebone boot sequence is sometimes being interrupted by noise in the form of characters during booting, as described by https://andicelabs.com/2014/07/beaglebone-black-boot-issues/ . I'm not sure this is the problem, but I am getting hangs when trying to reboot, particularly if it is a restart after ungracefully pulling out the power.
I've followed the instructions at the website above (and https://www.digikey.com/eewiki/display/linuxonarm/BeagleBone#BeagleBone-SetupmicroSDcard), and managed to compile u-boot.img and MLO. Then i'm not sure whether I can just copy MLO and u-boot.img onto the SD card boot partition (replacing the original ones from the bela 0.3.6a image) as suggested in the original post of the first website, or whether I need to manually write these to the beginning of the partition using dd as suggested in the second website (and in the comments of the first website). I tried the latter, though I wasn't sure if it made sense to do this on a partition, and it didn't work at all (messed with the partition table). I also tried just copying these files onto the boot partition (using the cp command when booted into the beaglebone from the emmc). This worked better, but got as far as saying "Uncompressing Linux Kernel ... done, booting the kernel" and just hangs.
I thought I could try using the bela_image_builder.sh, but wasn't sure how to edit the bootloader patch file. Any help would be most appreciated