ocd_flash_banks now returns empty list when no flash banks are configured...

ocd_flash_banks now returns empty list when no flash banks are configured instead of failing. Allows more orthogonal implementations of tcl code.

git-svn-id: svn://svn.berlios.de/openocd/trunk@1144 b42882b7-edfa-0310-969c-e2dbd0fdcd60