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>
diff --git a/doc/README.imx6 b/doc/README.imx6
index add1d80..0e00968 100644
--- a/doc/README.imx6
+++ b/doc/README.imx6
@@ -150,7 +150,8 @@
 After building, you need to create a command sequence file and use
 Freescales Code Signing Tool to sign both binaries. After creation,
 the mkimage tool outputs the required information about the HAB Blocks
-parameter for the CSF.
+parameter for the CSF. During the build, the information is preserved
+in log files named as the binaries. (SPL.log and u-boot-ivt.log).
 
 More information about the CSF and HAB can be found in the AN4581.
 https://cache.freescale.com/files/32bit/doc/app_note/AN4581.pdf