opkg: Don't call sync() when installing into an offline root

opkg: Don't call sync() when installing into an offline root

Even when installing onto a live target system, calling sync() during
package installation is of somewhat questionable benefit.  But calling
it on the build host during rootfs construction is certainly useless and
can cause I/O to stall for several seconds on even a moderately sized
host which is clearly not desirable.

(From a patch originally by Mike Crowe.)

(From OE-Core rev: f48a68177510e8f2d4fcc3725a6dfc41a9a8e96b)

Signed-off-by: Phil Blundell <philb@gnu.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>