classes/buildhistory: properly process escaped chars from pkgdata

classes/buildhistory: properly process escaped chars from pkgdata

All values written out to pkgdata are escaped (see write_if_exists() in
package.bbclass). In practice there tend not to be characters that need
escaping except in the scriptlets (pkg_preinst, pkg_postinst, pkg_prerm
and pkg_postrm) where currently we still see the escape codes in the
corresponding files within buildhistory (e.g. \n and \t) and thus also
in the output of buildhistory-diff, hindering proper diffing of changes.
To fix this, when we read values from pkgdata and write them out to
buildhistory, we need to interpret the escape codes by doing the exact
reverse of what we do in package.bbclass.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>