Use linker ASSERT statement to prevent undetected overlapping of

sections on PPChameleon board; other boards might use this, too.

board/dave/PPChameleonEVB/u-boot.lds

@@ -140,6 +140,7 @@ SECTIONS
    *(COMMON)
   }
 
+  ppcenv_assert = ASSERT(. < 0xFFFF8000, ".bss section too big, overlaps .ppcenv section. Please update your confguration: CFG_MONITOR_BASE, CFG_MONITOR_LEN and TEXT_BASE may need to be modified.");
   . = 0xFFFF8000;
   .ppcenv :
   {