fbset: Replace deprecated update-alternatives syntax

Building fbset gave the following warning:

	WARNING: fbset: Use of ALTERNATIVE_NAME is deprecated, see
		update-alternatives.bbclass for more info.

update-alternatives.bbclass states that this warning will eventually become an
error so this is worth fixing.

The use of deprecated syntax is replaced with current syntax to do the same
thing. To test that this works, a .deb package was created and the post-install
script was checked to ensure that update-alternatives would be called with the
exact same arguments as it was by the script generated by the old recipe.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>