ipk: Decode byte data to string in manifest handling

```
File: '/home/andrew/src/openbmc/openbmc/meta/lib/oe/package_manager/ipk/manifest.py', lineno: 69, function: create_full
     0065:        output = pm.dummy_install(pkgs_to_install)
     0066:
     0067:        with open(self.full_manifest, 'w+') as manifest:
     0068:            pkg_re = re.compile('^Installing ([^ ]+) [^ ].*')
 *** 0069:            for line in set(output.split('\n')):
     0070:                m = pkg_re.match(line)
     0071:                if m:
     0072:                    manifest.write(m.group(1) + '\n')
     0073:
Exception: TypeError: a bytes-like object is required, not 'str'
```

Change-Id: Ifefb13bfa22c766d20ab9f73f7abe5163b3df86f
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>