useradd: Fix build architecture corruption of sstate artefacts

When mixing aarch64 and x86_64 host builds with common sstate, useradd failures
were found. This was due to references to:

/home/pokybuild/yocto-worker/qemuarm/build/build/tmp/sysroots-components/aarch64/pseudo-native/usr/bin/pseudo
or
/home/pokybuild/yocto-worker/qemuarm/build/build/tmp/sysroots-components/x86_64/pseudo-native/usr/bin/pseudo

on the wrong architecture within the useradd postinst files.

Instead of using COMPONENTS_DIR as the FIXME, use PSEUDO_SYSROOT which contains
the arch component as well to avoid this.

Add extra comments to cause the relavent sstate to be rebuilt.

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