generic: enhanced image Makefile

* Creation of uImage for WNR854T only done once (before 2x for jffs2 build and 1x for squashfs build)
* Got rid of unneccessary padding of rootfs partition
* ARM zImages always need a machine id, therefore do not copy generic (=no id) uImage to BIN_DIR, instead copy zImage
* Generalized functions for easier re-using and enhancing (e.g. D-Link DNS 323 implementation would be only a couple lines)
* Copy rootfs partitions to BIN_DIR, just like it is done for D-Link DNS 323
* Use variables to allows easily changing for custom builds, e.g. kernel mtd size for symbols
* Size check of kernel files to avoid builds that break devices
* Use for "-sysupgrade" and "-factory" in image names (like ar71xx, brcm63xx, etc.) to avoid questions about which image to use

Signed-off by: Matthias Buecher <mail@maddes.net>

SVN-Revision: 33174