busybox: fix the on-target upgrade problem

busybox: fix the on-target upgrade problem

We now can have a 'one-binary' version of busybox, or 'two-binary'
version of busybox, controlled by the 'BUSYBOX_SPLIT_SUID' variable.
This makes on-target upgrade a problem, as we have to support the
following four upgrading paths.

For convenience, in the following context, A is used to denote a
'two-binary' version of busybox while B is used to denote a 'one-binary'
version of busybox.

A --(upgrade)--> B
B --(upgrade)--> A
A --(upgrade)--> A
B --(upgrade)--> B

This patch makes effort to support the above four situations.

[YOCTO #4802]

(From OE-Core rev: 4e571e97750f3ac6a62cd0d2d10c08be98230630)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>