px4: enable APPS_BINDIR support

useful for APM startup script
stdio: added vdprintf() for printf to a file descriptor
px4io: fixed non-test usage of servos

we need to advertise the armed flags, and disable lockdown on arming
ms5611: cope with a transient I2C bus error

if we fail to send a command to to the MS5611 then try it again in one
tick. Otherwise we end up collecting bad data.
ROMFS: add support for EXTERNAL_SCRIPTS

this adds support for an EXTERNAL_SCRIPTS directory, to complement the
EXTERNAL_APPS option. It allows external apps to install scripts in
ROMFS
if rc.APM is installed, run it