This patch introduced a bug preventing flash writes from working
on Cortex-M3 targets like the STM32.  Moreover, it's the wrong
approach for handling no-MMU targets.

The right way to handle no-MMU targets is to provide accessors
for physical addresses, and use them everywhere; and any code
which tries to work with virtual-to-physical mappings should use
a identity mapping (which can be defaulted).

And ... we can tell if a target has an MMU by seeing if it's
got an mmu() method.  No such methood means no MMU.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>