sdlog2: parameters logging

Write all parameters to log on startup. If parameter will be changed during log writing (i.e. when armed, if sdlog2 started with -a option) it will be NOT logged.

Message format for parameters (PARM) compatible with APM and uses "float" type for value. Looks that it's enough by now.

Some reorganization of message types: IDs >= 0x80 reserved for special "system" messages: format, timestamp, parameters, version. IDs < 0x80 used for buisness messages. Actually it doesn't matter for user.