Makefile: preserve output for images that can contain HAB Blocks

To being able to sign created binaries, we need to know the HAB Blocks
for that image. Especially for the imximage type the HAB Blocks are
only available during creation of the image. We want to preserve the
information until we get to sign the files.
In the verbose case we still get them printed out instead of writing
to log files.

Cc: sbabic@denx.de

v2-Changes:
 - No usage of MKIMAGEOUTPUT_$(@F) macro.
 - Predefine default value /dev/null in every involved Makefile.

Signed-off-by: Sven Ebenfeld <sven.ebenfeld@gmail.com>
Reviewed-by: George McCollister <george.mccollister@gmail.com>
Tested-by: George McCollister <george.mccollister@gmail.com>