package/parted: relax dependency on lvm2

parted can be configured without support for LVM volumes, so
we can relax the dependency on lvm2, and make it an optional
feature.

Remove now no-longer needed dependency on MMU because of lvm2.

(Based on a code-snippet from Thomas.)

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>