libopkg: specify "/bin/sh" instead of "sh"

libopkg: specify "/bin/sh" instead of "sh"

avoid strange behavior with execvp() when PATH is not set in environment
(in which case confstr(_CS_PATH) should return something reasonable)

reproducable running openwrt 15.05 and 15.05.1 and attempting to install
a software package (e.g. libuuid) via LuCI (prior to openwrt/luci#1048).
(https://github.com/openwrt/luci/pull/1048) libuuid.postinst fails with
status 255 on 15.05 and opkg segfaults in 15.05.1.  This probably merits
further exploration.

Originally reported in https://github.com/openwrt/packages/issues/1922

Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>