openocd: convert function setup_command_handler() to static

openocd: convert function setup_command_handler() to static

The function setup_command_handler() was used in the eCos build of
OpenOCD, thus it was exported and a comment was added to remark it
should remain not-static. Unfortunately the comment was missing
the relevant information of the special build that requires so.
Also unusual is that there is no include file that declares the
prototype of the function.

The comment above the function was added in two steps, in commit
ea3e49f4e22d ("fix embedded builds") and commit fb96b8607a0c
("openocd: setup_command_handler() must not be static"), again
without info about the special build.
Also the mailing list archive does not report any further detail.
The only hint is in the first commit above that also adds a test
on BUILD_ECOSBOARD in a #if.

Commit 39650e2273bc ("ecosboard: delete bit-rotted eCos code")
removes all the eCos code, that effectively includes the both the
prototype and the call to the function setup_command_handler(),
	http://openocd.zylin.com/#/c/503/2/src/ecosboard.c@a1092
but did not reverted the function to static.

With all the 'external' uses of this function being dropped, set
the function setup_command_handler() to static and remove the
obsoleted and misleading comment.

Change-Id: I4d6b83dec2a838119821189fc67949bfca070035
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5902
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>