external-python-tarball: code refactoring

external-python-tarball: code refactoring

external-python-tarball only can be built when inherit package_ipk now.
Update it to reuse existed populate sdk code that it could be built for
rpm and deb too.

Remove var DEPENDS, SDK_* and flags of do_populate_sdk because they are
already defined in populate_sdk bbclasses.

[Yocto 3006]

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>