kconfig: Use PATH_MAX instead of 128 for path buffer sizes.

This prevents the buffers being overflowed when using a config
file with a long name. PATH_MAX is used elsewhere in the same
file, so use it here as well.

Signed-off-by: Will Newton <will.newton@imgtec.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>