mavlink: fixed build warning
GPS: added comment on units for variance
build: cope better with absolute paths
build: allow for external apps via EXTERNAL_APPS

this is used by the APM build
Add FIONWRITE to allow applications to sniff the amount of writable space on a descriptor. Implement this for serial devices only.
serial: fixed up FIONREAD and FIONWRITE

the device ioctl returns -ENOTTY when it hasn't handled the command
added vdprintf() library function
added PWM_SERVO_SET_UPDATE_RATE ioctl

this allows apps to set the PWM update rate. APM needs this to honor
the user configurable PWM rate setting for copters.
fixup rate
fixed PWM_SERVO_GET

this was using the wrong channel
increase the UART buffer sizes to 256

The most critical one is the GPS serial port receive buffer size,
which needs to be at least 128 to support the UBLOX protocol, but it
seems a good idea for people running a FMU without a IO board to
increase the UART buffer sizes generally
Merge commit '63464a8959d90bd92c4998a80992c416b85c2057'
Merge commit '0fff8aa23b477bebda9c1a79f5cd0b5eceeb95d1'
Merge commit '890b1ac0689984cb6bc4638cc2aa8ec869d14f91'
Merge commit '920608bf3671662f94b4939ffdaf6fde2b08edff'
Merge commit '375d3c14d742248b434c080527886a95ea1d563f'
Merge branch 'master' of https://github.com/PX4/Firmware