Since using more than 256 mission items has been tested and fixed, the
limit can now be raised to more than 256 (but smaller than UINT16_MAX).

Since the whole file is allocated in advance on NuttX, the limit there
is (arbitrarily) set to 2000. For POSIX archs such as SITL, we don't
really need to constrain ourselves any longer.

The dataman started overwriting contents after 256 items because the
item index were only uint8_t. This fix allows for more than 256
waypoints.

AeroFC don't have a SD Card or any other storage device, so changing
dataman backend to work over RAM to be able to load missions.

altitude controlled modes

Signed-off-by: Roman <bapstroman@gmail.com>

The current ICM-20608 is nearing EOL.

I have tested on both ICM-20608 and ICM-20602 based Pixracers.

   Upstream nuttx needs to have the flash programing support added.