bridge-utils: use update-alternatives bbclass

This fixes a read-only-rootfs bug, as the manual update-alternatives command
being run didn't utilize ${D}, so failed at do_rootfs time.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #39 from kergoth/read-only-rootfs-fixes

bridge-utils: use update-alternatives bbclass