openocd: src: replace the GPL with eCos exception 2.0 license tag

Add the license exception text in the license pool.
Add the exception chapter in license-rules.txt
Replace the boilerplate with the SPDX tag.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: Ied513b7c9c0722ed2a9c11dbdff3fbf59f1b41ce
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7061
Tested-by: jenkins

LICENSES/exceptions/eCos-exception-2.0

@@ -0,0 +1,20 @@
+SPDX-Exception-Identifier: eCos-exception-2.0
+SPDX-URL: https://spdx.org/licenses/eCos-exception-2.0.html
+SPDX-Licenses: GPL-2.0-only, GPL-2.0-or-later
+Usage-Guide:
+  This exception is used together with one of the above SPDX-Licenses.
+  To use this exception add it with the keyword WITH to one of the
+  identifiers in the SPDX-Licenses tag:
+    SPDX-License-Identifier: <SPDX-License> WITH eCos-exception-2.0
+License-Text:
+
+As a special exception, if other files instantiate templates or use
+macros or inline functions from this file, or you compile this
+file and link it with other works to produce a work based on this
+file, this file does not by itself cause the resulting work to be
+covered by the GNU General Public License. However the source code for
+this file must still be made available in accordance with section (3)
+of the GNU General Public License.
+
+This exception does not invalidate any other reasons why a work based on
+this file might be covered by the GNU General Public License.