openocd: build: add SPDX tag

Add the SPDX tag to makefiles, configuration scripts and tcl files
present in the folders under src/

Change-Id: I1e4552aafe46ef4893d510da9d732c5f181784a4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7051
Tested-by: jenkins

src/target/Makefile.am

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 %C%_libtarget_la_LIBADD = %D%/openrisc/libopenrisc.la \
 	%D%/riscv/libriscv.la \
 	%D%/xtensa/libxtensa.la \