In the current multi image build process the DTBs end up uncompressed
in the PBL. This can be annoying because the PBL is often very size
constrained.
This patch allows to put the DTBs in as lzo compressed binary into
the PBL. Since lzo offers quite good compression ratios for DTBs no
other compression algorithm has been implemented for now.
Boards which want to use the compressed DTBs only have to change
the __dtb_ prefix in the DTB name to __dtb_z_. Also they should select
ARM_USE_COMPRESSED_DTB to make sure barebox supports uncompressing
the DTB.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>