opkg: enable libsolv backend by default

opkg: enable libsolv backend by default

The libsolv backend is vastly superior than the currently enabled
internal ad-hoc solver. While the switch does have a small impact on
disk and memory footprint, it make sense to change the default as for
most cases the disk/memory footprint hit should be acceptable.

========================
Disk Footprint Increase
========================
qemux86-64	 523K
qemuarm  	 445K
qemux86  	 576K

====================================================
Command [1]           Libsolv      Internal Solver
====================================================
opkg update          26.21 MB      26.21 MB
opkg list            29.87 MB      29.87 MB
opkg install procps  30.99 MB      27.33 MB
opkg remove procps    1.69 MB       1.69 MB
opkg update	     30.97 MB	   27.75 MB

[1] Profile done via 'valgrind --tool=massif <command>' in a feed with
~18K packages.

Signed-off-by: Alejandro del Castillo <alejandro.delcastillo@ni.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>