boards/b-l072z-lrwan1: enforce CPU reset when connecting with openocd

connect_assert_srst is used to enforce a CPU reset before connecting to
it with openocd. This is useful when debugging on-chip.

For details on openocd reset configuration, see

boards/b-l072z-lrwan1/dist/openocd.cfg

 source [find target/stm32l0.cfg]
 
-reset_config srst_only
+reset_config srst_only connect_assert_srst
 
 $_TARGETNAME configure -rtos auto