useradd.bbclass: Fix build time install issues

useradd.bbclass: Fix build time install issues

When the system attempts to populate the sysroot's passwd/group files, it
does so in a single block.  However, with the way it was previously
implemented, the system would always run through the code necessary to
populate the sysroot, even in the case of target packages.  This had
the side effect that a cross-installed filesystem may not match a
target installed filesystem.

The code was slightly reorganized to ensure that the cross/target installed
pre-install script behavior is the same.  It also moves the block that
configures the sysroot parameters to the sysroot specific section of
the code.

Also some minor validation was occuring even on nativesdk packages.
Nativesdk packages should be skipped when processing useradd ops.

(From OE-Core rev: 99fd0f14fd774c9194f62795e6023880e3aa5612)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>