The bird2 babel implementation has proven to be the more reliable option
over babeld, especially on low-end hardware. It has been working
flawlessly on many test installations.
Use bird2 instead of babeld, if no implementation is specified via uci.
While at it, use the automatically incrementing $(COMMITCOUNT) for
PKG_RELEASE.
Signed-off-by: Fabian Bläse <fabian@blaese.de>