python-smartpm: style fixes

python-smartpm: style fixes

* Use ${PN} instead of python-smartpm
* Use multi-line strings instead of multiple single line += (also
  avoiding the confusing "_append +=")
* Use class-native instead of virtclass-native - no difference in
  behaviour, but this keeps things consistent if we have to add
  any target overrides in future.

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