This adds the make infrastructure to build multiple SoC or
board specific images from a single barebox binary.

The basic idea is that we no longer have a single pbl, but instead
multiple pbls, one per image if necessary. Each pbl is defined
by its entry function so that each pbl can do exactly what a given
board needs. Additionally the pbls together with a self extracting
barebox binary can be encapsulated in specific image formats.

squashed in build fixes from Lucas Stach for make version >= 3.82:

Split Multimage Makefile rule in explicit and implicit parts

Fixes build with make version >=3.82

Frome the make 3.82 NEWS file:
* WARNING: Backward-incompatibility!
  In previous versions of make it was acceptable to list one or more explicit
  targets followed by one or more pattern targets in the same rule and it
  worked "as expected".  However, this was not documented as acceptable and if
  you listed any explicit targets AFTER the pattern targets, the entire rule
  would be mis-parsed.  This release removes this ability completely: make
  will generate an error message if you mix explicit and pattern targets in
  the same rule.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Lucas Stach <dev@lynxeye.de>