Fix shared-ng.inc inclusion with CWD other than the fmk install dir

[zardus]

Some of the fmk scripts (such as extract-multisquashfs-firmware.sh) don't chdir to the fmk directory, causing the shared-ng.inc script to fail when sourced, as it assumes that it's running in the fmk directory. This PR makes shared-ng.inc sourceable from anywhere.