sysbuild: Fix exporting BOARD.+ through sysbuild cache

Lately, sysbuild started filtering out variable names matching ^BOARD
during sysbuild cache file generation. As a result, one of the cache
variables that is no longer exported to other domains is BOARD_ROOT,
which breaks the recent support for out-of-tree boards in sysbuild.

Of those variables, the only one which is reasonable to filter out is
BOARD itself, because the purpose of that is to substitute a different
value for the one found in sysbuild's own CMake cache, which has the
board revision stripped out.

Update the relevant if-condition and use a single regex for brevity.

Fixes #59114.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>