/boot is a directory, copy files instead of replacing it with symlink

for the record, u-boot doesn't like having /boot/fit -> ../nix/store/..../fit
symlinks so we don't use symlinks inside /boot either

pkgs/systemconfig/default.nix

@@ -99,8 +99,10 @@ in attrset:
       cp -v -fP \$src/bin/* \$src/etc/* \$dest
       ${if attrset ? boot then ''
         (cd \$dest
-         if test -e boot ; then rm boot ; fi
-         ln -sf ${lib.strings.removePrefix "/" attrset.boot.target} ./boot
+         test -d boot || mkdir boot
+         cd boot
+         cp ../${lib.strings.removePrefix "/" attrset.boot.target}/* .
+         sync; sync
         )
       '' else ""}
       EOF