rootfs_ipk.bbclass: run preinst/postinst scripts with "-e"

rootfs_ipk.bbclass: run preinst/postinst scripts with "-e"

There are scripts that can and should work when being ran on build
host (for example, simple update-rc.d), but there are also many which
can't and won't ever (for example, anything adding users/groups).
The second group sometimes doesn't get "unpacked" flag because
scripts throw errors in the middle and return something nice from
the last command.

It can be considered as a bug in pre/postinst script (as it should
either explicitly check for "${D}" or just do "set -e" at start),
but it is common enough.

There is also another aspect to this as in general we can't be sure
that everything is OK wrt preinst/postinst if script commands throw
errors.

Running preinst/postinst scripts on host with "-e" should solve
that.

Signed-off-by: Roman I Khimov <khimov@altell.ru>
Acked-by: Chris Larson <clarson@kergoth.com>