rootfs generation: export two new variables to postinst scriptlets
In order for the postinst scriptlets to be able to run once we need to
export the location of the intercept scripts and also the location of
native sysrootfs. The gdk-pixbuf binaries will need the latter because
in order to generate the loaders.cache it will need to scan some shared
libraries that must be native. Even though the output is a text file.
