find_md5 sorts the list of package files with the locale of the user.
The top-level Makefile sets LC_ALL=C, but a make call in a package
directory, such as make -C package/kernel/linux, uses the locale of
the shell. With en_GB.UTF-8, some file names sort in a different
order, so the hash and the name of the prepared stamp differ, and the
package is built again. The next top-level build then builds it again.
Always sort in the C locale.
Signed-off-by: John Crispin <john@phrozen.org>