sys/util.h: Add IS_EMPTY and LIST_DROP_EMPTY macros

IS_EMPTY macro allows to check if defined name is empty, i.e.
does not contain replacement list.
LIST_DROP_EMPTY macro may be used to process __VA_ARGS__ type lists,
e.g. a,b,,c , and remove empty elements.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>