Netifrc/Brctl Migration

These days utility can manage bridges, it is superior than using the old specific utilities, among others is the 's brctl. Modern Linux expose bridge setting via sysfs, as result there is no need for iproute2 to support complex configuration as brctl utility, same sysfs configuration can be used for brctl based configurations as well.

brctl Migration to iproute2
The migration of brctl to iproute can be done in two phases:
 * 1) Migrate bridge configuration to sysfs, this can be done in stable.
 * 2) Migrate bridge management into iproute2 and drop brctl usage, this requires -0.4.0.

Once sysfs migration is completed, migration to iproute2 will be done as soon as netifrc supports iproute2, at this time can be safely removed from system.

Old sysfs keys
In the past, bridge and brport settings were specified as variables without a prefix, now one should specify bridge_ or brport_ prefix, for example:

Should be specified as:

Bridge Master Interface Settings
These are setting of the bridge master device, the name of interface is the bridge name.

Bridge Slave (Port) Settings
These are setting of the bridge slave device (port), the name of interface is the slave name.

stp
Becomes:

port
Becomes: