AP_HAL_SMACCM: remove meaningless example dir
ArduCopter Makefile: Don't include targets.mk directly (included by apm.mk)
Rover Makefile: don't include targets.mk directly (included by apm.mk)
Arduino.mk: more verbose default config file, with default HAL_BOARD apm2
AP_HAL_AVR: UARTDriver guard needs AP_HAL_Boards.h
AP_HAL_SMACCM: guard compilation on CONFIG_HAL_BOARD