lisa_shell: fix lisa-help command run from anywhere in the filesystem

Created by: mdigiorgio

When running lisa-help from directories other than the LISA home directory, the help information were not printed because the command was looking for LisaShell.txt in the current directory. This adds the absolute path to that file so that the help is printed from anywhere in the filesystem.

Signed-off-by: Michele Di Giorgio michele.digiorgio@arm.com