LU-17058 build: add help and checkpatch as make targets

LU-17058 build: add help and checkpatch as make targets

Add `make help` to print out available make targets. The
output is styled after the Linux kernel `make help`.
Add `make checkpatch` to run checkpatch.pl script
against most recent commit.

Update README to mention `make help`.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I65ce84040502994ae7caa0c8b72d808442f6b79e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52142
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>