busybox: prevent postinst script from failing

busybox: prevent postinst script from failing

recent addition in rootfs_ipk.bbclass which runs postinst scripts with
-e option, while very useful, prevents busybox from configuring at image
build time. It will be configured at first image boot if your file
system is writable, but in case of e.g. squashfs, that will be fatal
error. This commit prevents postinst to fail, so busybox will create
all needed links at image build time.