Raspberry Pi 2/3 GPIO UART do not work. How to activate?

Hi all,

I'm running alpine 3.6.2 on rpi2 and rpi3.
Now i'm trying to do a disk install. If i add root=something (with or without rootfstype=ext4) to the cmdline.txt, there are only a black screen at boot and no network (yellow LED blink and i can read dmesg from sdcard from another computer).
I've got a Bus Pirate which i use as USB-TTL adapter. This work perfectly with raspbian with my two rpi. But nothing with alpine.
Tried with console=ttyAMA0,115200 and console=ttyS0,115200.
Added enable_uart=1 in usercfg.txt
Tried with a fresh install of alpine with same result.

Is there somethng to do to activate UART on GPIO 14/15?

Thanks