Documentation: update SubmitChecklist for O=objdir and kconfig testing

Add build testing using 'O=builddir'.

Add build testing with various kconfig symbols disabled, listing
common ones that are known to cause build problems.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>