Install on Orange pi pc-plus

Hello,

I'm trying to install AlpineLinux on my OrangePi PC+ SBC.
I have downloading at http://dl-cdn.alpinelinux.org/alpine/v3.6/releases/armhf/alpine-uboot-3.6.2-armhf.tar.gz and flash a sd-card.
But the screen remains blank.
How can I select the device with extlinux.conf ?
Where to put extlinux.conf ? at the root of the 1st partition, I have just one ext4 partition
I can't find any howto except on the wiki but it's seems outdated.

Notice I have already install Armbian with success on this device.

Can somebody help me ?