cfg: change default SAM4L cortex_m reset_config

From testing this target does not seem to support using SYSRESETREQ, change
the default to the safe VECTRESET.

This target also has other reset issues (srst not working) that will be
addressed in another patch.

Change-Id: Icfc78347dc71aa3a062ddea63190a818d7fbc760
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1995


Tested-by: jenkins
Reviewed-by: Angus Gratton <gus@projectgus.com>

tcl/target/at91sam4lXX.cfg

@@ -6,3 +6,6 @@ source [find target/at91sam4XXX.cfg]
 set _FLASHNAME $_CHIPNAME.flash
 flash bank $_FLASHNAME at91sam4l 0x00000000 0 1 1 $_TARGETNAME
 
+# if srst is not fitted use VECTRESET to perform a soft reset
+# this will only reset the core, not the peripherals
+cortex_m reset_config vectreset