oe-core: add world-image

* this depends on newer bitbake
* similar to world target, but collects all PACKAGES
* recipes marked with EXCLUDE_FROM_WORLD are excluded from world as well
  as world-image
* recipes marked with EXCLUDE_FROM_WORLD_IMAGE are excluded completely
  from world-image, sometimes you can build 2 recipes in same sysroot,
  but cannot install them in the same image
* PACKAGES listed in EXCLUDE_PACKAGES_FROM_WORLD_IMAGE are also
  excluded. You can remove e.g. ${PN}-dbg, but keep all other PACKAGES.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
initrdscripts: exclude conflicting packages from world-image

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
sstatesig: introduce SignatureGeneratorOEBasicHashLite

* Rebuilds packages when:
  a) If you edit patches of a given recipe it will rebuild.
  b) If you change a task like do_configure, things after it will rerun in
     a given recipe.
  c) If a dependent task in another recipe changes, it won't rerun.
  d) If you change some global configuration variable or shared .bbclass
     it will rebuild.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>