[telemetry] reverse order of messages to send periodicaly

This way, messages that are send at low frequency have less chance to be
discarded if the uart buffer is filled by too many messages at the same
time. Higher frequency messages are most likely to find an empty slot.
[conf] fixedwing ahrs use gps course instead of mag for heading by default
[ahrs] warn if PERIODIC_FREQUENCY is lower than AHRS freqs
[config] better warning message and update example airframe
[plotter] open new plotter with parent settings
[plot] draw curves at low resolution when moving/selecting in log plotter

much faster when dealing with huge logs
[boards] add driver for KroozSD BigRotorcraftEdition

- IMU with memsic MXR9500 accelerometer
- set mag correct freq to 75Hz for krooz_sd_x

closes #586
[build] ARM toolchain/Makefile cleanup

- possible to explicitly pass a PREFIX to use other toolchain
  default for PREFIX is arm-none-eabi or arm-linux-gnueabi
- print meaninfull error if arm-linux toolchain is not found
- printcommands prints only CC and GCC version
- remove obsolete find OpenOCD stuff