fs: ensure $(TARGET_DIR_WARNING_FILE) is writable

If the Buildroot tree is read-only, then $(TARGET_DIR_WARNING_FILE) is
copied read-only into target/ but we may want to remove it during the
build process.

This poses no real problem, since target/ itself is guaranteed to be
writable, but for good measure, force $(TARGET_DIR_WARNING_FILE) to be
writable itself.

Reported-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>