modified .gitignore to ignore .vrx and .hex files
VRBRAIN: enabled the compilation of module bl_update for bootloader update from nuttx
VRBRAIN: added automatic update of bootloader from SD at startup
VRBRAIN: some not important adjustments
VRBRAIN: renamed external and on board drivers for MPU6000 and HMC5883 sensors
VRBRAIN: added filter dfu
AP_HAL_VRRBAIN: added comment
VRBRAIN: deactivated error on HMC5883 start
VRBRAIN: included module esc_calib for ESC's calibration from shell
VRBRAIN: deleted unnecessary customizations
VRBRAIN: changed the management of the pwm output
VRBRAIN: change default pin for analog input.