Thanks Pali for the exlanations!
I installed the power48 modules manually and the started the u-boot+kp48.bin using the flasher-3.5 on my laptop.
Runs fine now! So I probalby can flash the .bin without problems now

Only noticed I lost the /sys/class/power_supply/bq27200-0/uevent
file to get battery status info. Is this because of the I2C issues I read about in other threads/irc?

With your u-boot I probably would be able to play around booting kernels from other (sdcard) partitions. I'll play with that in the next weeks...