Implement flashing to a specific XMC4500 board via variable JLINKSER

Makefile

@@ -34,7 +34,14 @@ SIZE  = $(TOOLCHAIN)-size
 
 STACK_SIZE?=2048
 
-JLFLAGS = -device XMC4500-1024 -if SWD -speed 300
+JLINKSER?=0
+JLINKSERSTR=
+
+ifneq ($(JLINKSER),0)
+	JLINKSERSTR=-SelectEmuBySN ${JLINKSER}
+endif
+
+JLFLAGS = -device XMC4500-1024 -if SWD -speed 300 ${JLINKSERSTR}
 
 CFLAGS = -target armv6m--none-eabi -mcpu=cortex-m4 -mthumb -integrated-as -ccc-gcc-name arm-none-eabi-gcc
 # CFLAGS = -g