build: use mkhash for IPK metadata checksums

build: use mkhash for IPK metadata checksums

When setting the option IPK_FILES_CHECKSUMS the build system stores
checksums of all package file as metadata. In combination with pkg_check
this allows to see if a package is broken, e.g. caused by bad flash.

To create those checksums the tool `sha256sum` were used while the rest
of OpenWrt uses `mkhash`, a small & fast implementation of sha256. As
the build system does not check the existence of `sha256sum` and the
stderr output is moved to /dev/null, a situation where the option is
enabled but no actual checksum are created may occur.

Instead of adding `sha256sum` as a requirement, this replaces it with
`mkhash sha256` and adapts the `sed` pipe command to fit spacing.

CC: Xu Wang <xwang1498@gmx.com>
CC: Michal Hrusecky <Michal@Hrusecky.net>
Signed-off-by: Paul Spooren <mail@aparcar.org>