PX4 build fixes

This makes some progress towards aligning APM PX4 build with the latest PX4; however, we still get some link errors:

...
LINK:    /home/vagrant/PX4-Firmware/makefiles//build/firmware.elf
/home/vagrant/PX4-Firmware/makefiles//build//tmp/ArduCopter.build/module.pre.o: In function `PX4::PX4Scheduler::reboot()':
/home/vagrant/ardupilot/libraries/AP_HAL_PX4/Scheduler.cpp:173: undefined reference to `up_systemreset()'
/home/vagrant/PX4-Firmware/makefiles//build//home/vagrant/PX4-Firmware/src/drivers/px4io/module.pre.o: In function `PX4IO::mixer_send(char const*, unsigned int)':
/home/vagrant/PX4-Firmware/src/drivers/px4io/px4io.cpp:1258: undefined reference to `mavlink_vasprintf'
/home/vagrant/PX4-Firmware/src/drivers/px4io/px4io.cpp:1262: undefined reference to `mavlink_vasprintf'
/home/vagrant/PX4-Firmware/makefiles//build//home/vagrant/PX4-Firmware/src/drivers/px4io/module.pre.o: In function `PX4IO::init()':
/home/vagrant/PX4-Firmware/src/drivers/px4io/px4io.cpp:558: undefined reference to `mavlink_vasprintf'
/home/vagrant/PX4-Firmware/src/drivers/px4io/px4io.cpp:419: undefined reference to `mavlink_vasprintf'
/home/vagrant/PX4-Firmware/src/drivers/px4io/px4io.cpp:446: undefined reference to `mavlink_vasprintf'
/home/vagrant/PX4-Firmware/makefiles//build//home/vagrant/PX4-Firmware/src/drivers/px4io/module.pre.o:/home/vagrant/PX4-Firmware/src/drivers/px4io/px4io.cpp:536: more undefined references to `mavlink_vasprintf' follow
make[1]: *** [/home/vagrant/PX4-Firmware/makefiles//build/firmware.elf] Error 1
make[1]: Leaving directory `/tmp/ArduCopter.build'
make: *** [px4] Error 2