scripts/gdb: add cmdline reader command

lx-cmdline Report the Linux Commandline used in the current kernel

[jan.kiszka@siemens.com: remove blank line from help output and fix pep8 warning]
Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>