lvm2: make it more explicit that readline support is disabled by default

Any PACKAGECONFIG default assigned with ??= is lost by subsequent
assignments with +=, therefore lvm2 is currently building with the
readline PACKAGECONFIG disabled. Make that explicitly clear in the
recipe. Also minor formatting updates, to align with style guide.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>