checkpatch: add --kconfig-prefix

Kconfig allows to customize the CONFIG_ prefix via the $CONFIG_
environment variable.  Out-of-tree projects may therefore use Kconfig with
a different prefix, or they may use a custom configuration tool which does
not use the CONFIG_ prefix at all.  Such projects may still want to adhere
to the Linux kernel coding style and run checkpatch.pl.

One example is OP-TEE [1] which does not use Kconfig but does have
configuration options prefixed with CFG_.  It also mostly follows the
kernel coding style and therefore being able to use checkpatch is quite
valuable.

To make this possible, add the --kconfig-prefix command line option.

[1] https://github.com/OP-TEE/optee_os



Signed-off-by: Jerome Forissier <jerome@forissier.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Joe Perches <joe@perches.com>
Link: http://lkml.kernel.org/r/20200818081732.800449-1-jerome@forissier.org


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>