simulator: Fully simulate external storage.

The external storage will be created during make install, as simext folder in
the build directory. Upon pressing the e key the sim will mount (virtually
) this into the root directory. It can be accessed in the same way as an
sd/mmc card on real targets. This requires quite some path trickery in io.c.

Change-Id: I2fa9070a3146101ec5655b5b4115ca349d1d4bf4
4 files changed