build: handle directory with whitespace in AUTOREMOVE clean

build: handle directory with whitespace in AUTOREMOVE clean

Package with whitespace in their build directory are not correctly
removed when CONFIG_AUTOREMOVE is enabled. This is caused by xargs that
use whitespace as delimiters. To handle this use \0 as the delimiter and
set find to use \0 as the delimiter.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>