checkpatch: check for OpenOCD tree, not for kernel tree

checkpatch: check for OpenOCD tree, not for kernel tree

checkpatch.pl looks for Linux kernel specific paths and files
to check source tree. As openocd misses kernel files it ends
with this error message:

	Must be run from the top-level dir. of a kernel tree

This patch also renames 'kernel' -> 'openocd'
in source tree-related messages.

This is the old commit c5d89883165e ("checkpatch.pl: check for
openocd tree, not for kernel tree") re-applied.
Also remove the flag "--no-tree" from .checkpatch.conf, not
required anymore.

Change-Id: I336a66558c75494b7ae339ea63559c31f23aad84
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/5122
Tested-by: jenkins