After all was done, it finally did no longer boot anywhere but into recovery -- no matter which recovery I flashed. It was still possible to boot into fastboot mode (Volume down + Power button), then select "normal system boot", but that's certainly not a good user experience on every power-on.
Additionally, the "charge battery when powered off" image was no longer working: plugging in power would also boot into recovery.
Some googling finally lead me to a xda-developers forum post which has the solution: there is a raw partition in the flash, which apparently stores the default boot option for the boot loader, just wiping this partition will restore the default boot order.
So when booted into recovery (must have adb enabled), just run
adb shell \
dd if=/dev/zero \
of=/dev/block/platform/msm_sdcc.1/by-name/miscfrom your computer (adb installed and USB cable connected, of course).
This should fix booting (it did for me).