elflink: Use CurrentDirName for initial PATH if valid

elflink: Use CurrentDirName for initial PATH if valid

commit c250a42263c8 ("elflink: Set PATH to the directory containing
ldlinux.c32") made the mistake of thinking that CurrentDirName doesn't
contain useful data for PXELINUX, it may do. If a user specifies a
Path Prefix (DHCP option 210) we enter load_env32() with that path as
the current working directory. If that path is thrown away instead of
being used for PATH we may fail to load a config file as specified
with Configuration File Option (DHCP option 209).

Signed-off-by: Matt Fleming <matt.fleming@intel.com>