arm7_9: Avoid infinite loops in bulk write dispatching

arm7_9: Avoid infinite loops in bulk write dispatching

Add a mandatory field in struct arm7_9_common for regular, non-optimized
memory writes. Together with the existing bulk_memory_write field, this
allows variants to select any combination of implementations for regular
and bulk writes, without risking infinite loops from accidentally using
bulk writes for implementing bulk writes.

ARM 7/9 targets may now select arm7_9_memory_write_opt as their
target.write_memory implementation, which will dispatch to
arm7_9_common.bulk_write_memory if possible, or fallback to
arm7_9_common.write_memory otherwise.

To avoid loops, bulk write implementations mustn't call any other
functions than arm7_9_write_memory_no_opt() to write memory; it will
unconditionally call arm7_9_common.write_memory. If they fail, they should
simply return error to allow the caller to fallback to regular writes.

Tested on a regular ARM7TDMI only.

Change-Id: Iae42a6e093e2df68c4823c927d757ae8f42ef388
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1685
Tested-by: jenkins
Reviewed-by: Sergey A. Borshch <sb-sf@users.sourceforge.net>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>