zephyr-flash-pyocd.bbclass: support for flashing via pyocd

Implement do_flash_usb for boards supported by pyocd:

    MACHINE=xxx bitbake yyy -c flash_usb

Pyocd support abundance of boards, however for now this
meta-layer supports only one board that can be flashed
using pyocd, that is 96Boards Nitrogen.

Describe the feature in README.

Signed-off-by: Wojciech Zmuda <wojciech.zmuda@huawei.com>
Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>

README.txt

@@ -43,6 +43,29 @@ The same sample, for Nios2 image:
     $ MACHINE=qemu-nios2 bitbake zephyr-philosophers
     $ runqemu qemu-nios2
 
+Flashing
+=================================
+
+You can flash Zephyr samples to boards. Currently, the following MACHINEs
+are supported:
+ * DFU:
+  - arduino-101-sss
+  - arduino-101
+  - arduino-101-ble
+ * pyocd:
+  - 96b-nitrogen
+
+To flash the example you built with command e.g.
+
+    $ MACHINE=96b-nitrogen bitbake zephyr-philosophers
+
+call similar command with explicit flash_usb command:
+
+    $ MACHINE=96b-nitrogen bitbake zephyr-philosophers -c flash_usb
+
+dfu-util and/or pyocd need to be installed in your system. If you observe
+permission errors or the flashing process seem to hang, follow those instructions:
+https://github.com/pyocd/pyOCD/tree/master/udev
 
 Building and Running Zephyr Tests
 =================================

classes/zephyr-flash-pyocd.bbclass

@@ -0,0 +1,16 @@
+python do_flash_usb() {
+    from pyocd.core.helpers import ConnectHelper
+    from pyocd.flash.file_programmer import FileProgrammer
+
+    image = f"{d.getVar('DEPLOY_DIR_IMAGE')}/{d.getVar('PN')}.elf"
+    bb.plain(f"Attempting to flash {image} to board {d.getVar('BOARD')}")
+
+    with ConnectHelper.session_with_chosen_probe() as session:
+        FileProgrammer(session).program(image)
+        session.board.target.reset()
+}
+
+addtask do_flash_usb
+
+do_flash_usb[nostamp] = "1"
+do_flash_usb[vardepsexclude] = "BB_ORIGENV"

conf/machine/96b-nitrogen.conf

@@ -4,4 +4,5 @@
 #@DESCRIPTION: Machine configuration for 96Boards Nitrogen Board.
 
 require conf/machine/include/nrf52832.inc
+ZEPHYR_INHERIT_CLASSES += "zephyr-flash-pyocd"
 ARCH_96b-nitrogen = "arm"