cmake: Don't lose global sysbuild value with zephyr_get(... MERGE)

According to the doc comment:

  If using MERGE then SYSBUILD GLOBAL will get both the local and global
  sysbuild scope variables (in that order, if both exist).

This stopped working in commit 8460d91e,
when support for `zephyr_get(... VAR <multiple-variables>)` was added.

Instead of returning both values, the local sysbuild scope value would
clobber the global one. Fix this by splitting the internal `sysbuild`
scope into `sysbuild_local` and `sysbuild_global`, in that order.

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