I managed to some more output to the serial log (booted with "console=ttyS1,115200" only, no "console=tty0"):
starting version 219
[ 1.851984] sd 2:0:0:0: [sda] Assuming drive cache: write through
/init: line 307: chroot: not found
Target filesystem doesn't have requested /sbin/init.
/init: line 325: chroot: not found
/bin/sh: 0: Can't open splash
[ 2.909334] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00
I managed to some more output to the serial log (booted with "console= ttyS1,115200" only, no "console=tty0"):
starting version 219
[ 1.851984] sd 2:0:0:0: [sda] Assuming drive cache: write through
/init: line 307: chroot: not found
Target filesystem doesn't have requested /sbin/init.
/init: line 325: chroot: not found
/bin/sh: 0: Can't open splash
[ 2.909334] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00