This problem is still not solved, so I started over with a fresh clean Ubuntu Xenial (next) image. Even with a new installation I get the same error messages for brcmfmac, but this time it started as expected...
But after apt-get upgrade / dist-upgrade and installing some software (apache2, zsh, logwatch, fail2ban, ...) it brings me back to the same problem during the boot process:
It just boots to the emergency mode after 100 seconds.
I already checked the file system of my MultiMediaCard but fsck -pvcf /dev/sdc1 and dumpe2fs -b /dev/sdc1 did not show any bad blocks and did not solve the problem.
:-( Any advice?