base-files: Add function pi_include the /lib/functions/boot.sh used by...

base-files: Add function pi_include the /lib/functions/boot.sh used by preinit, which safely sources files as well as working correctly whether the script using it is a package added to the jffs2 part of a squashfs or built into an image or on some other root filesystem.

SVN-Revision: 23099