boards: arm: mps2_an521/musca: kconfig: Clean up BOARD defaults

Putting 'if's directly on the defaults is simpler here.

I'm guessing BOARD should always be "musca_{a,b1}_nonsecure" if it isn't
"musca_{a,b1}", so I removed the condition on the
"musca_{a,b1}_nonsecure" default (turning it into an "else").

Avoiding a top-level 'if'/'depends on' also avoids adding direct
dependencies to the BOARD symbol, which looks a bit neater in the
generated docs (though direct dependencies only matter for symbols that
might be selected/implied).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>