Merged in alinjerpelea/nuttx (pull request #942)

tools: add flash_writter for cxd56 boards

this flash writter is using the xmodem for firmware transfer

for flashing please use
tools/flash_writer.py -s -c /dev/ttyUSB0 -d -b 115200 -n nuttx.spk

Signed-off-by: Alin Jerpelea <alin.jerpelea@sony.com>

Approved-by: Gregory Nutt <gnutt@nuttx.org>

tools/README.txt

@@ -1025,3 +1025,12 @@ zipme.sh
   I use this script to create the nuttx-xx.yy.tar.gz tarballs for
   release on Bitbucket.org.  It is handy because it also does the
   kind of clean that you need to do to make a clean code release.
+
+flash_writter.py
+----------------
+
+  This flash writter is using the xmodem for firmware transfer on
+  boards based on cxd56 chip (Ex. Spresense)
+
+  for flashing the .spk image to the board please use:
+  tools/flash_writer.py -s -c /dev/ttyUSB0 -d -b 115200 -n nuttx.spk