rtos: add support for NuttX

This patch introduces RTOS support for NuttX. Currently,
only ARM Cortex-M (both FPU and FPU-less) targets are supported.

To use, add the following lines to ~/.gdbinit.

define hookpost-file
  eval "monitor nuttx.pid_offset %d", &((struct tcb_s *)(0))->pid
  eval "monitor nuttx.xcpreg_offset %d", &((struct tcb_s *)(0))->xcp.regs
  eval "monitor nuttx.state_offset %d", &((struct tcb_s *)(0))->task_state
  eval "monitor nuttx.name_offset %d", &((struct tcb_s *)(0))->name
  eval "monitor nuttx.name_size %d", sizeof(((struct tcb_s *)(0))->name)
end

And please make sure the above values are the same as in
src/rtos/nuttx_header.h

Change-Id: I2aaf8644d24dfb84b500516a9685382d5d8fe48f
Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>
Signed-off-by: Masatoshi Tateishi <Masatoshi.Tateishi@jp.sony.com>
Signed-off-by: Nobuto Kobayashi <Nobuto.Kobayashi@sony.com>
Reviewed-on: http://openocd.zylin.com/4103
Tested-by: jenkins
Reviewed-by: Alan Carvalho de Assis <acassis@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>