[rotorcraft] lisa/lia baro configuration: e.g. <configure name=LISA_M_BARO value=BARO_SPI/> in airframe file
Merge pull request #367 from scdwyer/master-fix_stm32_spi

Fix for SPI driver to handle different lengths of tx and rx buffer.
[imu_aspirin] should now work for lpc21 with SPI as well...
untested, please report, see the microjet_twog_aspirin.xml as possible example
[firmwares] rotorcraft: separate modules "task", fw/rc: if PRINT_CONFIG, show some more configured frequencies
[sys_time] some performance improvements
stm32 now uses AHB_CLK / 8 as clock input for systick
[sys_time] cleanup and get_sys_time_float() inline function
get_sys_time_usec for stm32, overflows after 70min (same as before)
currently only used in SysTimeTimer, which is only used at startup AND
the sys_mon module, so that module doesn't work anymore after 70min!
[sys_time] cleanup: use sys_time.nb_x instead of old cpu_time_x
[stm32][spi] Handled zero length tx and non zero rx transfers more cleanly so a tx buffer is not required. Tx buffer of length equal or greater than rx buffer still required if tx length > 0.