swconfig: Link with libubox

swconfig: Link with libubox

Fixes linking failures observed with external toolchains:

/home/florian/dev/toolchains/stbgcc-4.8-1.5/bin/../lib/gcc/mipsel-linux-gnu/4.8.5/../../../../mipsel-linux-gnu/bin/ld:
warning: libubox.so, needed by
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so,
not found (try using -rpath or -rpath-link)
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blobmsg_open_nested'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blobmsg_parse'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blob_nest_end'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libuci.so:
undefined reference to `blobmsg_add_field'

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
(cherry picked from commit fe8618a8fe0db1bc8f343c0f75082ff96e9991ab)