added one more stub, Simon's BASIC does not crash anymore

STATUS.md

@@ -76,6 +76,7 @@ Not all of them - only these we want to have implemented.
 | Address   | Name         | Status   |  Remarks                                           |
 | :-------: | :----------- | :------: | :------------------------------------------------: |
 | `$A004`   | revision str | DONE     |                                                    |
+| `$A408`   | REASON       | NOT DONE |                                                    |
 | `$A453`   | (unknown)    | NOT DONE |                                                    |
 | `$A533`   | LINKPRG      | DONE     |                                                    |
 | `$A644`   | new          | NOT DONE |                                                    |

c64/basic/a408.interop

@@ -0,0 +1,2 @@
+
+This address is called at least by the Simon's BASIC cartridge.

c64/basic/a408.reason.s

@@ -0,0 +1,5 @@
+
+// XXX provide documentation and implementation
+
+REASON:
+    STUB_IMPLEMENTATION()

c64/kernal/fe25.memtop.s

@@ -17,7 +17,7 @@ MEMTOP:
 
 	// FALLTROUGH
 
-memtop_set:
+memtop_set: // for compatibility reasons this has to start at $FE2D
 
 	sty MEMSIZK+1
 	stx MEMSIZK+0