kbuild: make sorting initramfs contents independent of locale

Some LANG values (e.g. pl_PL.UTF-8) cause the sort command to output
files before their parent directories, which makes them inaccessible for
the kernel. In other words, when the kernel populates the rootfs, it is
unable to create files whose parent directories have not been yet created.

This patch makes sorting use the default (LANG=C) locale, which results in
correctly laid out initramfs images (parent directories before files).

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>