tools: rmboard.py: Fix conversion from run_pipe to new helper

tools: rmboard.py: Fix conversion from run_pipe to new helper

When this utility was converted from run_pipe and to the new output
helper, two problems were introduced. First, the conversion for calling
"git rm -f" wasn't correct. Change this to match the other conversions.
Second, the final call we do we need to construct the list because we
print that command for the user to use to inspect remaining references.

Fixes: 3d094ce28a22 ("u_boot_pylib: Add a function to run a single command")
Signed-off-by: Tom Rini <trini@konsulko.com>