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 <[email protected]>
CC: Michal Hrusecky <[email protected]>
Signed-off-by: Paul Spooren <[email protected]>
Paul Spooren