Linux to posix conversion

Changed "linux" target to "posix". Most of the changes are shared with
QuRT and with OSX. The Linux specific parts are in for i2c which uses
<linux/i2c.h> and <linux/i2c-dev.h>.

There is also a check for __PX4_LINUX in mavlink for a tty ioctl that is
not supported.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>