fix: grep not match interface listed by `ip link list`

Interfaces listed by `ip link list` are prefixed with the index
identifier. The pattern "^$BRNAME" does not match.

 - dependencies to ifconfig and ip removed
 - wait until interface flagged with IFF_UP

Ref: https://github.com/torvalds/linux/blob/master/include/uapi/linux/if.h

Signed-off-by: Joshua Brunner <j.brunner@nexbyte.com>