[oe] Generating slightly different images for different IMAGE_FSTYPES

[Martin Jansa]

Hi,

on om-gta01 we had problem that even our minimal image
shr-lite-image is too big for its 64MB NAND chip

We're slowly stripping packages not really needed, but it would be 
nice to remove whole /boot with kernel-image.

Problem is that when we're booting on gta01, than /boot/uImage is used so 
it should be installed in .tar.gz image. But when booting from NAND, then
kernel is usually in different NAND partition and /boot on rootfs partition
is useless.

In patch is what I used now to generate images like this, but it's not optimal at all
1) It copies whole rootfs 3 times if you have all jffs2/ubi/ubifs in FSTYPES, 
   instead just once for -no-boot version (it's possible ie if we expect certain order 
   of making FSTYPES or prepare -no-boot always with normal rootfs and don't remove it
2) task-base still depends on kernel-image and I don't see how to easily remove that 
   dependency, without defining different image recipe and building it separately. 
   So with next kernel upgrade user will get new kernel-image which will probably fail
   to install because of lack of space.

Is there better way to solve it or should we try harder to save space somewhere else?