cmake: add nuttx force-upload target (#9717)

If the PX4FMUv2 board does not have the v5 uploader, it's not possible
to check the silicon revision to make sure it's safe to upload the
firmware which is more than 1 MB. However, if the user is sure the
silicon revision is not affected by this errata, he can upload the
firmware using px4_uploader.py with --force argument. This commit adds
`force-upload` make target to do it more convenient way.

Signed-off-by: Andrei Korigodski <akorigod@gmail.com>