Wic throws this message when any of the build artifacts are
not provided:
Build artifacts not completely specified, exiting.
(Use 'wic -e' or 'wic -r -b -k -n' to specify artifacts)
It was not clear which artifact was not specified.
Reworked the code to specify list of missed artifacts.
Now the message looks like this:
The following build artifacts are not specified:
bootimg-dir, kernel-dir, native-sysroot
[YOCTO #7912]
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
