[mavlink-socket] Add mavlink-socket application

This application exposes a mavlink connection from a serial port with a
UDP socket.  It is useful for controlling a Gumstix AeroCore board from
a DuoVero board.  Credit goes to Andrew Smith [1] for putting this
recipe together.

[1] https://github.com/smithandrewc/meta-stanfordARL

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Correct typo in ROOTFS_POSTPROCESS_COMMAND

There must be a semi-colon after each command to ensure each gets
individually executed.

Signed-off-by: Ash Charles <ashcharles@gmail.com>