[Nuttx] Don't bail to debug shell if no SD card present

[jaxxzer]

This is a mildly frequent issue that comes up. I am doubtful of the utility of this 'feature'. Does anyone make regular use of it? I feel like using a serial adapter on UART5 or dropping to nsh via mavproxy is a very reasonable tradeoff to bring down the support issues and confused users when they find themselves in this situation. Also not sure if this happens in chibios.

[jaxxzer]

See here for particular line in rc script: https://github.com/bluerobotics/ardusub/issues/112

[jaxxzer]

Add a tune when there is no sd card at boot

[Naterater]

The tune here, right? http://ardupilot.org/copter/docs/common-sounds-pixhawkpx4.html

[WickedShell]

This was done in 5c4b5c7