target: require working area for physical/virtual addresses to be specified

Fixed bug: if virtual address for working memory was not specified
and MMU was enabled, then address 0 would be used.

Require working address to be specified for both MMU enabled
and disabled case.

For some completely inexplicable reason this fixes the regression
in svn 2646 for flash write in arm926ejs target. The logs showed
that MMU was disabled in the case below:

https://lists.berlios.de/pipermail/openocd-development/2009-November/011882.html



Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>