cfg: or1k_generic: allow TAP_TYPE override

This patch allows users to specify TAP_TYPE from
the command line when using or1k_generic.cfg.

Change-Id: I9f4b7d8e4867658fa34bb4e92fc3a5f25227df11
Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Reviewed-on: http://openocd.zylin.com/2056
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

tcl/board/or1k_generic.cfg

@@ -4,7 +4,10 @@
 set FPGATAPID 0x020b30dd
 
 # Choose your TAP core (VJTAG , MOHOR or XILINX_BSCAN)
-set TAP_TYPE VJTAG
+if { [info exists TAP_TYPE] == 0} {
+   set TAP_TYPE VJTAG
+}
+
 # Set your chip name
 set CHIPNAME or1200