Fixes: #25656
The commit a8e90cbfb3 improved handling of
shields. Unfortunately it pops items from the SHIELD variable which
works well if SHIELD is a CMake ;-separated list, but if the list is
a space separated list, as may be provided by the user, then this
principle fails as reported in #25656.
This is fixed by keeping improved behaviour, while popping from an
alternative SHIELD-NOTFOUND list instead.
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
8d3e822e79