bitbake: build.py: create separate function for shell trap creation code

Currently, the shell trap code was created in exec_func_shell(). Split
the function so that we can create the code separately.

Also, some whitespaces were automatically deleted by my editor. Since
this is not necessarily a bad thing, leave these changes too.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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>