checkpatch: fix for flag --no-tree

checkpatch: fix for flag --no-tree

When checkpatch is run with command line flag --no-tree, it cannot
find local 'companion' files and has to skip loading them.

This has caused issues with change https://review.openocd.org/7211
on jenkins.

Skip loading 'tools/scripts/camelcase.txt' with flag --no-tree.
While there, rewrite the associated error message.

Change-Id: I6ede7b16f9ccd77b9118fd9be7ada07a1ac96952
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7212
Tested-by: jenkins