fetch2: enable checksum definition as SRC_URI parameter

fetch2: enable checksum definition as SRC_URI parameter

URI parameters should be able to be defined as a parameter of the SRC_URI,
this patch enables thus for checksums.

An example;

SRC_URI = "http://pkgconfig.freedesktop.org/releases/pkg-config-${PV}.tar.gz;md5sum=a3270bab3f4b69b7dc6dbdacbcae9745;sha256sum=3ba691ee2431f32ccb8efa131e59bf23e37f122dc66791309023ca6dcefcd10e"

Addresses the remainder of [YOCTO #1399]

(Bitbake rev: 5f8f923b76722c9b6c7ffbe19e94df50f900155f)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>