PX4 Firmware plugin: First try to switch flight mode, then arm if successful - which is a lot safer than arming and then seeing if we can switch modes.

Firmwareplugin: Trim retry period to something that matches closer the users attention and is still long enough to allow for multiple heartbeats to pass through

The ardupilot parameter meta data generator will generate dups when the
parameter is in ifdef'ed code

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>