parse: Return IOError when including file with absolute path

* resolve_file was behaving different when relative and absolute
  paths were passed to it

* include relative-path/non-existent-file.inc
  works correctly resolve_file throws IOError, BBHandler.py:handle()
  doesn't catch it, ConfHandler.py:include() catches IOError and shows:
  DEBUG: CONF file 'relative-path/non-existent-file.inc' not found
* include /absolute-path/non-existent-file.inc
  was failing, because resolve_file just returns fn,
  BBHandler.py:handle() calls bb.parse.mark_dependency(d, abs_fn)
  which throws:
  OSError: [Errno 2] No such file or directory: '/absolute-path/non-existent-file.inc'
  and parsing fails.
  Ad isfile() test for absolute fn and throw IOError to make
  resolve_file behavior consistent for both paths.

* I know we had some issues with -b relative-path-to-recipe.bb and
  absolute path, so consider this patch only as RFC and documentation of
  this problem

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
world-image: add extra target

* 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>