RPi: just use RPI instead of RPI2.

The reason for this change is that RPi2 and RPi3 are compatible, and
hopefully all differences coming up can be resolved without ifdefs but
at runtime.

posix-configs/rpi2/mainapp.config

@@ -1,27 +0,0 @@
-uorb start
-param set SYS_AUTOSTART 4001
-param set MAV_BROADCAST 1
-sleep 1
-param set MAV_TYPE 2
-df_lsm9ds1_wrapper start -R 4
-#df_mpu9250_wrapper start -R 10
-#df_hmc5883_wrapper start
-df_ms5611_wrapper start
-gps start -d /dev/pts/0
-sensors start
-commander start
-attitude_estimator_q start
-position_estimator_inav start
-land_detector start multicopter
-mc_pos_control start
-mc_att_control start
-mavlink start -u 14556 -r 1000000
-sleep 1
-mavlink stream -u 14556 -s HIGHRES_IMU -r 50
-mavlink stream -u 14556 -s ATTITUDE -r 50
-mavlink start -d /dev/ttyUSB0
-mavlink stream -d /dev/ttyUSB0 -s HIGHRES_IMU -r 50
-mavlink stream -d /dev/ttyUSB0 -s ATTITUDE -r 50
-navio_sysfs_rc_in start
-navio_sysfs_pwm_out start
-mavlink boot_complete