althold fix and acc scale improvement

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