scripts: target_config_lib: prepend target default package list from openwrt/tmp/.targetinfo

Device-specific package additions could generate `CONFIG_PACKAGE_...=m`
lines, which would override `CONFIG_PACKAGE_...=y` lines inserted by
OpenWrt for default packages (as Gluon did not know about these default
packages). This resulted in the unintended removal of such packages from
other devices that did not contain the same package in their device
package lists.

Avoid this issue by explicitly adding OpenWrt's target default package
list to the front of Gluon's target package list.