x86: generalize partition discovery for sysupgrade

x86: generalize partition discovery for sysupgrade

Generalize the partition discovery in sysupgrade in order to fix sysupgrade
and config backup/recovery on MMC block devices which use a different naming
scheme compared to mtdblock or sd* devices.

The change also adds the find applet to the ramdisk utilities so that upgrade
code can rely on it.

The commit is based on the initial submission by Russell Senior at
http://patchwork.ozlabs.org/patch/625440/ .

Signed-off-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>