Sometimes (but not that often) the dtb for a SPECIFIC device is appended to kernel,
but I doubt this is the case here
My pbp is setup this way (still)
using dd, copy 1st sector (mbr) somewhere, give name like emmc.mbr
copy 1st 16 MB somewhere (named as 1st-16.emmc.$DISTRO)
use something like dd if=/dev/emmc of=~/1st-16.emmc.manj bs=1M count=16,,,,, change emmc as appropriate
(that is emmc = mmcblk1 OR mmcblk2 OR maybe mmcblk0)
blank,,, dd if=/dev/zero of=/dev/emmc bs=1M count=16
restore mbr,,,,, dd if=emmc.mbr of=/dev/emmc (emmc.mbr should only be 512 bytes
(unless GPT ,, use 34 sectors,,,, dd default bs is 512 bytes so only need count)
So, if you check with fdisk /dev/emmc should all show correctly
Rest of 1st 16 M is /dev/zero,, blank,,,, so only uboot on SD can be used, it is the only one
If you want to restore just dd 1st-16M back to emmc
---edit---
I have never seen (unless very old) an arm device without dtb,
except for specific devices only, appended to kernel,,
so I think the gentoo is broken
but I doubt this is the case here
My pbp is setup this way (still)
using dd, copy 1st sector (mbr) somewhere, give name like emmc.mbr
copy 1st 16 MB somewhere (named as 1st-16.emmc.$DISTRO)
use something like dd if=/dev/emmc of=~/1st-16.emmc.manj bs=1M count=16,,,,, change emmc as appropriate
(that is emmc = mmcblk1 OR mmcblk2 OR maybe mmcblk0)
blank,,, dd if=/dev/zero of=/dev/emmc bs=1M count=16
restore mbr,,,,, dd if=emmc.mbr of=/dev/emmc (emmc.mbr should only be 512 bytes
(unless GPT ,, use 34 sectors,,,, dd default bs is 512 bytes so only need count)
So, if you check with fdisk /dev/emmc should all show correctly
Rest of 1st 16 M is /dev/zero,, blank,,,, so only uboot on SD can be used, it is the only one
If you want to restore just dd 1st-16M back to emmc
---edit---
I have never seen (unless very old) an arm device without dtb,
except for specific devices only, appended to kernel,,
so I think the gentoo is broken