Make sure sbin paths are in $PATH

The sbin paths (/sbin, /usr/sbin, /usr/local/sbin) need to be in $PATH.
Since $PATH is set in the build environment from the host, if the host
does not have these paths set, they need to be added to $PATH.  In
particular, they must be set for apt-get which calls out to ldconfig,
start-stop-daemon, etc.

The previous check was insufficient because if you had /usr/sbin or
/usr/local/sbin in your $PATH, the addition to $PATH was not done since
the "if" statement evaluated to true.

Change-Id: I52b41d2eb9794e7ee0ae600215cb3ceca191582f
Merge "Make sure sbin paths are in $PATH"