avr32: optimize image generation

 - remove the kernel+rootfs partitions from the MTD partition map,
 - use a temporary directory for intermediate image files,
 - append the root squashfs directly to the kernel,
 - don't hardcode uImage size in the jffs2 images,

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38194