esirisc: support eSi-RISC targets

esirisc: support eSi-RISC targets

eSi-RISC is a highly configurable microprocessor architecture for
embedded systems provided by EnSilica. This patch adds support for
32-bit targets and also includes an internal flash driver and
uC/OS-III RTOS support. This is a non-traditional target and required
a number of additional changes to support non-linear register numbers
and the 'p' packet in RTOS support for proper integration into
EnSilica's GDB port.

Change-Id: I59d5c40b3bb2ace1b1a01b2538bfab211adf113f
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/4660
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>