Finally, I also succeeded. As hopeless act I tried an image I has never used before
(Fedora-KDE-armhfp-28-20180325.n.0-sda.raw.xz). And it works. By mistake I changed
/etc/systemd/system/default.target by wrong way, but RPi was able to launch KDE GUI. It is
not so spry as on PC but it works. When I have changed default.target link to multi-user,
it boots into text mode. Perfect! The only essential thing which is still not working is
ethernet. I hope it will be solved in some next releases, without the functionality I can
do "nearly nothing" on my RPi. But following the steps on the way to booting RPi
3+ was for me very instructive, I learned a lot of. Thank You both, guys!
At the end, for potential followers, I wrote down all the necessary steps, how to prepare
SD card before you insert it into your RPi 3+. It is valid for now, for Fedora versions,
which are available nowadays, for some stable versions, which will come in the future, it
will be valid no more (I guess0. It is know-how I acquired from the thread.
fedora-arm-image-installer
--image=Fedora-KDE-armhfp-28-20180325.n.0-sda.raw.xz --target=rpi3 --media=/dev/mmcblk0
--norootpass --resizefs --selinux=off
mount /dev/mmcblk0p2 /tmp/sd2;mount /dev/mmcblk0p4 /tmp/sd4
cp -p /tmp/sd2/dtb-4.16.0-0.rc4.git0.1.fc28.armv7hl
/bcm2837-rpi-3-b.dtb
bcm2837-rpi-3-b-plus.dtb /tmp/sd2/dtb-4.16.0-0.rc4.git0.1.fc28.armv7hl
/bcm2837-rpi-3-b-plus.dtb
> rm -f /tmp/sd4/etc/systemd/system/default.target
> ln -s /lib/systemd/system/multi-user.target
/tmp/sd4/etc/systemd/system/default.target
> umount /dev/mmcblk0p2;umount /dev/mmcblk0p4