get_current_dir_name is a glibc extension, thus replace it
with a call to standard POSIX pathconf/malloc/gecwd.

The result slightly differs, because get_current_dir_name consults
the $PWD environment variable as as well, but that's ok, as it's
just an error message.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>