SCRIPT: add add_script_search_dir cmd

Add a add_script_search_dir cmd so that adding search
dir's can be added to cfg scripts.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>

doc/openocd.texi

@@ -590,6 +590,7 @@ Configuration files and scripts are searched for in
 @enumerate
 @item the current directory,
 @item any search dir specified on the command line using the @option{-s} option,
+@item any search dir specified using the @command{add_script_search_dir} command,
 @item @file{$HOME/.openocd} (not on Windows),
 @item the site wide script library @file{$pkgdatadir/site} and
 @item the OpenOCD-supplied script library @file{$pkgdatadir/scripts}.
@@ -5396,6 +5397,10 @@ Redirect logging to @var{filename};
 the initial log output channel is stderr.
 @end deffn
 
+@deffn Command add_script_search_dir [directory]
+Add @var{directory} to the file/script search path.
+@end deffn
+
 @anchor{Target State handling}
 @section Target State handling
 @cindex reset