initramfs-kexecboot-image: zap LDCONFIGDEPEND

* Override the settings of image.bbclass:
* we don't need ld.so.cache in our minimal image.
* While there, remove now unused ONLINE_PACKAGE_MANAGEMENT var.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>