[gcs] specify max map zoom level with -maps_zoom option
- defaults to 18, max is 22
  - TODO: check that a valid zoomlevel was given
  - add it to the menu to change at runtime?
- if available, larger (low zoomlevel) tiles in cache are loaded right away,
  even if higher zoomlevel maps could be downloaded.
[fixedwing] define default AHRS_PROPAGATE_FREQUENCY and AHRS_CORRECT_FREQUENCY in code instead of makefiles
[imu_umarim] set default AHRS_PROPAGATE_FREQUENCY to 50Hz in makefile
[fixedwing] print used ahrs freqs, pass configure options on...
[airborne] only print config messages at compile time if PRINT_CONFIG is defined
[rotorcraft] make RC_UPDATE_FREQ configurable
[dfu] match Lia as well as Lisa/M
[luftboot] update submodule to bump version to 1.1
[luftboot] minor makefile cleanup
[luftboot] readme update
[rotorcraft] lisa/lia baro configuration: e.g. <configure name=LISA_M_BARO value=BARO_SPI/> in airframe file
[stm32][spi] Fix for SPI driver to handle different lengths of tx and rx buffer. Note that tx buffer length MUST be greater than or equal to the rx buffer length in each transaction. Attempt to solve Issue #348. Also fixed DMA transfer length for using 16bit spi data lengths, untested. Some blocking WHILE loops that maybe need to be fixed.
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
[luftboot] doh, update submodule with fixed typo
[gps] resurrect the gps time sync using the sys_time ticks (works on lpc and stm32)
this is e.g. used for the external trigger sync
The resolution now depends on SYS_TIME_RESOLUTION
If real cpu ticks resolution is needed, a better arch independent solution is needed.