package/binutils: switch to use 2.29.x as the default version

Now that binutils 2.30 has been released, it is time to move to
binutils 2.29.x as the default binutils version, instead of 2.28.1.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>