kconfig: allow kconfig to handle longer path names

The current (arbitrary) limit of 128 characters for path names has
proven too short for Android builds, as longer path names are used
there.

Change conf.c, so it can handle path lengths up to PATH_MAX characters.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Michal Marek <mmarek@suse.com>