NuttX: the adapt to new implementation of ROMFS mounting at /etc

NuttX: the adapt to new implementation of ROMFS mounting at /etc

The option CONFIG_NSH_ROMFSETC has been replaced
by CONFIG_ETC_ROMFS and file system is mounted
during system initialization (sched/init/nx_bringup.c)
instead of application and NSH (nshlib/nsh_romfsetc.c).

The adapted OMK wrap-up code can be found in

  https://github.com/ppisa/nuttx-devel/tree/master/nuttx-omk-template/wrapup

It allows to wrap set of applications and define common
filesystem content in the final single binary including
even NuttX kernel.

Signed-off-by: Pavel Pisa <ppisa@pikron.com>