rootfs: remove ldconfig auxiliary cache where appropriate

rootfs: remove ldconfig auxiliary cache where appropriate

Removes the /var/cache/ldconfig auxiliary cache directory from
the rootfs when:
 - read-only-rootfs is in IMAGE_FEATURES,
 - ldconfig is not in DISTRO_FEATURES.

In both cases the /var/cache/ldconfig/aux-cache is useless.

Signed-off-by: Damian Wrobel <dwrobel@ertelnet.rybnik.pl>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>