shadow-sysroot: Simplfy login.defs installation

shadow-sysroot: Simplfy login.defs installation

Currently if shadow-sysroot is installed from a sstate package,
shadow_sysroot_sstate_postinst is looking in ${D} for login.defs which
isn't a valid for an install from sstate.

The easiest and most correct way to fix this is to override the standard
sysroot_stage_all function to install the files correctly and then
sstate installs work as expected. This simplifies the code as an added
bonus.

(From OE-Core rev: 6437e7315a0f2e077f9d69e4b65b48280ea8edbc)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>