classes/lib/useradd: The option -P is deprecated

This is also covered in documentation since:
https://git.yoctoproject.org/yocto-docs/commit/?id=3f3e5574ac9801ad92940168b61b532e0bd53a80



[YOCTO 14605]

Signed-off-by: Daiane Angolini <daiane.angolini@foundries.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>