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>

src/modules/systemlib/param/param.h

@@ -396,7 +396,7 @@ struct param_info_s {
 // See https://llvm.org/bugs/show_bug.cgi?format=multiple&id=18006
 // The following hack is for GCC >=4.8 only. Clang works fine without
 // this.
-#ifdef __PX4_LINUX
+#ifdef __PX4_POSIX
 				__attribute__((aligned(16)));
 #else
 				;