adapter: add command "adapter [de]assert srst|trst [[de]assert srst|trst]"

adapter: add command "adapter [de]assert srst|trst [[de]assert srst|trst]"

Inspired from http://openocd.zylin.com/#/c/3720/1

Add commands to control the adapter's signals srst and trst.
Add macros for the flag's values assert/deassert to make clear what
they mean and to propose a uniform set of values across the code.

Change-Id: Ia8b13f4ded892942916cad7bda49540a896e7218
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5277
Tested-by: jenkins