image*.bbclass, bootimg.bbclass: add image type dependencies
The following dependencies were manually added in the image creation
code. However, in order to have an image dependency mechanism in place,
use a new variable, IMAGE_TYPEDEP, to declare that an image type depends
on another being already created.
The following dependencies are added by this commit:
elf -> cpio.gz
live -> ext3
vmdk -> ext3
iso -> ext3
hddimg -> ext3
This commit adds also another new variable: IMAGE_TYPES_MASKED. Currently,
masking out certain types from IMAGE_FSTYPES was hardcoded in the image
creation code.
