Today I had a nice discussion with someone on the Fedora IRC channel about a problem booting from an USB stick with an iSCSI root filesystem and a bridged interface. He was facing the problem that the brctl tool was not available at boot time. CentOS and Fedora are using dracut to have everythink in place you need at boot time. For this use case brctl missed and he was not able to boot his machine.
In his special case he needed to setup the bridge at boot time because his root filesystem needs the connection not to be reconfigured and he needed a bridge device for his later KVM virtualization stuff on that machine. A dedicated second iSCSI interface wasn't an option.
After reading some dracut documentation I came over a blog post from a Russian which showed how to add the missing brctl binary to the initramdisk and make thinks work.
To include brctl into the initrd for your current kernel run:
dracut -I /sbin/brctl --force
That should create a working initrd with brctl to use the bridge configuration of dracut.
Here you can find the complete forum entry to see how the grub config has to look like to boot it from iSCSI on CentOS7: