Switch ubihealthd implementation for backported upstream version

We currently add a version of ubihealthd based on an early submission to the mtd-utils mailing list. There is now a different version of this tool that has been accepted into the mainline project.

Drop the existing version and backport the version provided in later releases of mtd-utils.

Signed-off-by: Martyn Welch martyn.welch@collabora.com