ShellPkg-ShellApp: Provide fully-qualified path to shell scripts

ShellPkg-ShellApp: Provide fully-qualified path to shell scripts

Provide fully-qualified path to shell scripts

Section 3.6.2 of version 2.2 of the shell specification requires that
the first positional argument (i.e. arg 0) of a shell script resolves
to "the full path name of the script itself."

Ensure that the startup script and any scripts launched by the shell
meet this requirement.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jim Dailey <jim_dailey@dell.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>