marking some more stuff const that shouldn't be changing. thx Hydra for the catch.

git-svn-id: https://afrodevices.googlecode.com/svn/trunk/baseflight@384 7c89a4a9-59b9-e629-4cfe-3a2d53b20e61
althold fix

acc now used for vel estimation and altitude estimation additionally to
baro.
mpu6050 sensitivity increased so acc_1G is now 4096 for newer chips.
added tunable parameters:
baro_cf_vel           //sets how much acc is used for velocity
estimation
baro_cf_alt            //sets how much acc is used for altitude
estimation
acc_unarmedcal  //if set to 1 accz_offset is allways updated when
unarmed
changed acc hardware to default, should be mpu6050 anyway