lib/oe/image.py: Fix dependency handling for compressed types

lib/oe/image.py: Fix dependency handling for compressed types

The dependency code needs to also include the dependency of base
types. For example:

 - sdcard.gz image with ext4

The dependency chain needs to include:

 - sdcard
 - ext4
 - gz

Until this change, the ext4 dependency were not being taken into
account when using the compressed one.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>