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.
