fetch2: improve usability of checksums

fetch2: improve usability of checksums

This patch improves the usability of checksums by only requiring one checksum
be defined.

Further, checksum verification will provide as much information as possible
at, rather than a bit at a time. No longer will you need to run fetch, see an
md5sum mismatch, fix it, run fetch, seen an sha256sum mismatch, fix it and
fetch again. If neither checksum is defined we now report both missing sums
at once - rather than one after the other.
Finally, if both sums are incorrect, we'll report both incorrect sums at the
same time.

Fixes part of [YOCTO #1399]

(Bitbake rev: 30ca41e596955fcbbe1ae6f4436ecf14cef96f8d)

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