cmake: resolved list issue when parsing shield list

Fixes: #25656

The commit a8e90cbf 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>